fix(mobile): add a native watchdog so a dead terminal document can't stay silently blank

CodeRabbit round: if the webview document dies before the glue can post
anything (or the RN message bridge never comes up), no error message and
no native handler fires. Arm a 15s foreground-gated watchdog per document
generation that paints the fatal overlay when web-ready never arrives;
first fatal diagnostics win over later cascades. Extract the watchdog and
the public contract types to keep TerminalWebView under the line cap, and
document the SVG xmlns percent-encoding transform.

Co-authored-by: Orca <help@stably.ai>

mobile/scripts/build-terminal-webview-engine.mjs

@@ -85,6 +85,10 @@ async function main() {
     readFile(require.resolve('@xterm/xterm/css/xterm.css'), 'utf8'),
     ...packages.map(readPackageVersion)
   ])
+  // Why: the no-external-URL regression gate bans http(s):// anywhere in the
+  // terminal document. These xmlns URIs live inside data: URLs (never fetched);
+  // percent-encoding the scheme colon satisfies the gate and URI-decodes back
+  // before the SVG is parsed.
   const engineCss = rawEngineCss
     .replace(/\/\*[\s\S]*?\*\//g, '')
     .replace(/http:\/\/www\.w3\.org\/2000\/svg/g, 'http%3A//www.w3.org/2000/svg')