Make Linux a supported platform: HTML context menus + runtime deps + release target

`bun run start` died on Linux with ERR_DLOPEN_FAILED (missing
libayatana-appindicator3.so.1). Fixing the environment exposed the real gap:
electrobun's ContextMenu.showContextMenu is a documented no-op on Linux, and
since the UI suppresses the webview's own right-click menu, EVERY panel context
menu was silently dead there — Send to Other Panel, Open, Rename, Permissions,
Delete, Copy Path, Reveal and the column picker all unreachable. Electrobun
itself logs "Application menus are not supported on Linux. Implement menu UI in
your webview HTML instead." at boot.

- Context menus are now HTML on EVERY platform (ui/src/app/context-menu.ts +
  context-menu.service.ts), not just as a Linux fallback: one implementation
  cannot drift from a second one. The item builders are ported verbatim from
  the old bun menu, so ordering, per-platform labels and the Windows-local
  chmod hide are unchanged, and picks are emitted as the same `ctx-*` / `col:*`
  strings through RpcService.emitMenuAction — App.handleMenuAction is untouched
  and still the single dispatcher for both the macOS menubar and right-clicks.
  Adds what the native menu gave for free: arrow/Enter/Escape keys, viewport
  edge-flip, close on blur/resize/scroll. The Edit item now shows the real F4
  hint instead of the native menu's "e" mnemonic.
- New shellItemAction RPC replaces showRowMenu/showColumnsMenu — the only part
  the webview can't do itself (reveal, open-with-default, copy path). Deletes
  src/bun/context-menu.ts entirely.
- release.ts gains a linux-x64 target. Electrobun emits only the .tar.zst
  updater blob on Linux, so the artifact is a .tar.gz of the assembled app
  folder, mirroring the Windows portable zip.
- Docs: platform-notes gets a Linux runtime-dependency table (AppIndicator is
  linked HARD — a missing lib is a boot failure, not a degraded tray), the
  ldd diagnostic, GNOME tray caveat, known issues and a first-boot smoke test;
  gotchas gets the ERR_DLOPEN_FAILED trail and the benign GLX/GBM startup
  noise; architecture/build-and-release/README updated accordingly.

Verified: backend `tsc --noEmit` clean, `bun run build:ui` clean (399 kB
initial), app boots on Linux with no errors beyond the documented benign
warnings. NOT verified on macOS/Windows — the native menu is retired on those
platforms too, so the menus need eyeballing on a Mac before the next release.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/gotchas.md

@@ -48,7 +48,30 @@ touching the adjacent code.
 - Tray: never call `setImage` after construction on macOS — the template
   treatment is lost and the icon renders black on dark menu bars.
 - `ApplicationMenu` is macOS-only in our usage; accelerators on Windows are
-  single-character only. ContextMenu exists on macOS+Windows, not Linux.
+  single-character only.
+- **`ContextMenu.showContextMenu` is a silent no-op on Linux** (electrobun's own
+  `dist/api/bun/proc/linux.md` says so; the app also prints "Application menus are
+  not supported on Linux" at boot). Because the UI suppresses the webview's own
+  right-click menu, that left EVERY panel context menu dead on Linux — no error,
+  nothing happens. The menu is plain HTML now (`ui/src/app/context-menu.ts`) on
+  every platform; don't reintroduce the native one for "nativeness" on mac/win
+  without keeping a Linux path.
+
+## Linux
+
+- **`ERR_DLOPEN_FAILED` on `bun run start`** means a missing system library, not
+  broken app code. `libNativeWrapper.so` links `libayatana-appindicator3.so.1`
+  *hard* (for the tray), so the app cannot start at all without it — even though
+  the tray is a minor feature. Diagnose with
+  `ldd build/dev-linux-x64/*/bin/libNativeWrapper.so | grep 'not found'`; the
+  `libasar.so => not found` line in that output is a red herring (it ships beside
+  the wrapper and resolves at load). Install list per distro is in
+  [platform-notes.md](platform-notes.md).
+- Benign startup noise on Linux — all non-fatal, the app runs normally:
+  `X11 Error: GLXBadWindow (code 170)`, `Failed to create GBM buffer of size
+  WxH: Invalid argument` (compositor/GLX chatter from the GTK webview), and
+  `libayatana-appindicator is deprecated. Please use libayatana-appindicator-glib`
+  (upstream electrobun's choice, not ours).
 - **Window-open flash prevention is three-layer** — remove any one and the
   white/inverted blink returns: (1) windows are created `hidden: true` and
   revealed by `showWhenReady` on the webview's first `dom-ready` (800 ms
@@ -100,9 +123,10 @@ touching the adjacent code.
   single-quoted entries** (`'…c'est…'`). Use double quotes for any value
   containing an apostrophe. Plurals are `{ one, other }` maps (not ICU strings);
   zh/ja supply only `other`.
-- **Native menus can't use the Angular service** — `menu.ts`/`context-menu.ts`
-  go through backend `i18n.ts`, and the app menu must be rebuilt when the locale
-  changes (wired in index.ts `settingsChanged`), or it keeps the old language.
+- **Native menus can't use the Angular service** — `menu.ts` goes through backend
+  `i18n.ts`, and the app menu must be rebuilt when the locale changes (wired in
+  index.ts `settingsChanged`), or it keeps the old language. The context menu no
+  longer has this problem: it is Angular now and reads `I18nService` directly.
 - `eu`/`zh`/`ja` are AI drafts pending native review (see the i18n README).
 - **Silent auto-refresh must snapshot sessionId+cwd and discard a mismatched
   result.** A normal `refresh()` is masked by `busy` (input blocked), but the