fix(build): explain missing mobile/ deps instead of failing build:mobile-web (#24325)

mobile/ is a separate pnpm project, so the root pnpm install leaves
mobile/node_modules empty and pnpm build failed in build:mobile-web with
hundreds of unexplained esbuild "Could not resolve" errors.

build-mobile-web-app-bundle now checks for mobile/node_modules first and
exits with the one command that fixes it. The probe path is shared with
the bundle tests' dependency check. CONTRIBUTING documents the install in
Local Setup and points "Before Opening a PR" at it.

Fixes #24324

Co-authored-by: Neil <neil@stably.ai>

.github/CONTRIBUTING.md

@@ -53,6 +53,15 @@ pnpm install
 pnpm dev
 ```
 
+`mobile/` is a separate pnpm project with its own lockfile, so the root `pnpm install` leaves its
+dependencies uninstalled. `pnpm build` (`build:mobile-web` fails without them) and the lint
+passes that cover `mobile/` need them. Install them before building or linting, and again
+when `mobile/pnpm-lock.yaml` changes:
+
+```bash
+pnpm -C mobile install --frozen-lockfile
+```
+
 ### Dev Profiles and Parallel Instances
 
 `pnpm dev` in a linked Git worktree uses its own userData profile, renderer port, and remote
@@ -170,7 +179,7 @@ Avoid vague names like `test`, `misc`, or `changes`.
 
 ## Before Opening a PR
 
-Run the same checks that CI runs:
+Run the same checks that CI runs, after the `mobile/` install from [Local Setup](#local-setup):
 
 ```bash
 pnpm lint