feat(windows): add native backdrop material presets (Mica / Acrylic /… (#412)

* feat(windows): add native backdrop material presets (Mica / Acrylic / Blur)

Adds a Background material dropdown (Auto / Blur / Mica / Mica Alt /
Acrylic / Off) that maps onto the native Windows backdrop APIs already
provided by the gpui fork — Mica and Mica Alt via
DwmSetWindowAttribute(DWMWA_SYSTEMBACKDROP_TYPE), Acrylic via the new
DWMSBT_TRANSIENTWINDOW material, and Blur via the classic
ACCENT_ENABLE_ACRYLICBLURBEHIND path — with no fork changes required.
* config: introduce WindowBackdrop in tty7-core with lenient kebab-case
  deserialization, defaulting to Auto for existing configs
* theme: resolve the backdrop through a build-number fallback chain
  (Mica/Mica Alt need Windows 11 22H2, Acrylic needs 22H2 natively and
  1809 via classic acrylic, Blur needs 1809; older builds fall back to
  plain translucency) and default the background alpha to
  SYSTEM_MATERIAL_OPACITY (0.82) while a material is active
* settings: replace the blur toggle with a localized backdrop dropdown
  that only lists the presets the current Windows build actually
  supports, and keep the settings panel fully opaque so workspace
  translucency never shows through it
* theme: make the file sidebar and right detail panel follow the window
  opacity so the backdrop material shows through the whole workspace,
  keeping row-level accents opaque for readability
* i18n: add backdrop keys for en, zh-CN and ja-JP, covered by the
  translation completeness test

* feat(theme): let the sidebar and right panel follow the window opacity

* update GPUI

* fix(windows): gate the sidebar translucency to translucent windows and sync the opacity slider

fix(windows): gate the sidebar translucency compensation to active materials

* fix(windows): derive the material opacity default from the resolved appearance

* fix(theme): keep WindowBackdrop semantics consistent on non-Windows

f

* fix(theme): stop Windows-only materials from pinning the blur on other platforms

* docs(changelog): document the Windows backdrop material settings

* refactor(theme): share the default window-opacity derivation

* fix(ui): keep gradient presets behind the settings panel and scope its fallbacks

* fix(ui): keep the settings theme picker legible and the backdrop label honest

f

* fix(theme): let every backdrop variant defer to the local blur toggle on non-Windows

* fix(settings): restore the backdrop dropdown selection on locale refresh

* fix(ui): keep the opened-file editor surface opaque under window translucency

* fix(settings): rebuild backdrop options after selection

* fix(settings): ignore synced windows backdrop overrides on other platforms

* fix(settings): preserve synced windows backdrop on non-windows reset

* fix(diff): keep the full-window overlay background opaque

* fix(windows): keep Auto opaque and stop the backdrop from misreporting itself

Ten findings from a review of the backdrop-material work, all in the
Windows-only paths.

The root one: `material_active` treated `Auto` as a material whenever the
legacy blur toggle happened to be on. `Auto` is the default in every config
written before this setting existed, and plenty of them carry
`window_blur: true` from the switch that no longer renders on Windows, so an
untouched install would drop from opaque to 0.82 alpha - with its file
sidebar and right panel at 0.15 - on first launch after the update, with no
visible control to undo it. Only an explicit pick in the dropdown now buys
the translucent defaults. The switch comes back on Windows while the
backdrop is `Auto`, since that is exactly when the legacy flag still decides
something.

The rest:

- Mica and Mica Alt fell back to `Blurred` with no lower bound, asking for a
  blur that does not exist below 1809 - and build 0, which is what a failed
  `RtlGetVersion` reports. They now degrade to plain translucency like
  `Blur` and `Acrylic` already did.
- Acrylic is no longer offered below 22H2, where it resolves to the very
  same classic WCA blur as `Blur`. A test now asserts that no two offered
  presets render identically on any build.
- `reload_from_config` re-applied the theme and the opacity slider but not
  the backdrop dropdown, so an external config change switched the window's
  material while the control kept naming the old one.
- The settings, opened-file and diff overlays were made opaque so the OS
  backdrop cannot show through their text; that also hid the theme
  background image, which used to show through them. They paint their own
  copy of it now, and the fill they share moved into
  `theme::overlay_background`.
- The SFTP transfers tray painted `workspace_surface_color` inside the right
  panel, which already paints it, stacking the same translucent surface
  twice into a darker band with a hard seam.
- `apply_theme` re-issued `set_background_appearance` on every `Config`
  mutation in every window. With a DWM material that now costs a
  `SetWindowPos(SWP_FRAMECHANGED)` frame recalc, so dragging the opacity
  slider recalculated the frame once per mouse sample; it is skipped when
  the appearance is unchanged.

* fix(ui): dim the overlay background image, and stop telling Windows it is macOS

Two defects found while driving the previous commit's changes in the app.

The overlays repaint the theme background image over their own opaque fill,
so it survives them being made opaque - but nothing dimmed it. Before those
overlays were opaque the image reached the eye through their translucent
fill; painting it at full strength put the settings text straight on top of
the wallpaper and made the panel unreadable at any image opacity above about
half. They now paint the image and then the workspace's own fill over it,
which is exactly the strength the image had through these overlays before,
and which needs no new constant to say so. Shared as
`app::overlay_surface_layers`, empty when the theme has no image so a
themeless window paints no second pass of anything.

The Windows-only blur row reused `SettingsBlurDesc`, whose text ends in
"(macOS)". It gets its own key in all three locales, describing the job the
flag actually still has on Windows: feeding the `Auto` material.

---------

Co-authored-by: l0ng-ai <24760907+l0ng-ai@users.noreply.github.com>

src/ui/code_editor.rs

@@ -918,12 +918,20 @@ impl Tty7App {
                 .absolute()
                 .inset_0()
                 .occlude()
-                .bg(cx.theme().background)
+                // Opaque on purpose: this overlay covers the whole workspace
+                // (everything but the detail panel) and an open file must never
+                // let the window translucency / backdrop material show through
+                // it. The preset's gradient fill is preserved, just with
+                // alpha 1 — the same paint the settings overlay uses. The
+                // theme background image is repainted on top of it, since the
+                // root's copy now sits below this fill.
+                .bg(crate::ui::theme::overlay_background(cx))
                 .on_key_down(cx.listener(|this, ev: &gpui::KeyDownEvent, window, cx| {
                     if ev.keystroke.key == "escape" {
                         this.toggle_code_panel(window, cx);
                     }
                 }))
+                .children(crate::ui::app::overlay_surface_layers(cx))
                 .child(h_flex().flex_1().min_h_0().w_full().child(editor_col))
                 .child(self.render_code_status_bar(window, cx))
                 .into_any_element(),