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/architecture.md

@@ -10,7 +10,8 @@ Dense reference for development and debugging. Companion files:
   its native launcher. Owns every privileged capability: filesystem, SSH,
   window management, native menus/dialogs/tray, the connection/settings stores.
 - **Webviews** (`ui/` Angular): one per window, system WebKit on macOS /
-  WebView2 on Windows. Zero privileged access — everything goes through RPC.
+  WebView2 on Windows / WebKitGTK on Linux. Zero privileged access — everything
+  goes through RPC.
 - **RPC transport**: one WebSocket server in the bun process, multiplexed by
   `webviewId` (each webview auto-connects using natively injected globals).
   Requests are typed by the `AppRPC` schema in `src/shared/rpc.ts`.
@@ -148,10 +149,6 @@ would float over editor windows).
 - `store/connections.ts` — plaintext-passwords JSON (deliberate v1 choice),
   atomic tmp+rename, 0600 + win32 `icacls` ACL. `store/settings.ts` — defaults
   spread-merge on load, so NEW SETTINGS KEYS need no backend change.
-- `context-menu.ts` — native row/empty-space menus + the column picker.
-  Local shell actions (reveal/open/copy-path) run bun-side via Utils; others
-  forward through `menuAction` and the UI applies them to the row it
-  remembered (`FileActionsService.menuTarget`). Labels are per-platform.
 - `menu.ts` — macOS-only app menu, rebuilt on every connections change (the
   Connect submenu mirrors the store). ⌘W is intercepted by the menu, so bun
   routes `close-tab` to the focused editor window first. Labels go through
@@ -161,7 +158,8 @@ would float over editor windows).
   locale (from `settings.locale`, `system` via `Intl…resolvedOptions().locale`);
   `t(key, params)` shares the same `src/shared/i18n` dictionaries as the UI.
   Wired in index.ts on boot and on `settingsChanged` (relocalizes + rebuilds
-  the app menu). `context-menu.ts` labels resolve through `t()` at show-time.
+  the app menu). The context menu no longer needs this — it is Angular and reads
+  `I18nService` directly.
 - `tray.ts` — static icon, click = activate main. macOS template image;
   Windows colored icon. No `setImage` calls ever (they lose the template
   treatment) and no menu (clicks arrive directly).
@@ -220,12 +218,24 @@ would float over editor windows).
   boot and on the `settingsChanged` broadcast (so editor/manager windows follow).
 - `file-actions.service.ts` — toolbar/keyboard/context-menu file ops, always
   against an explicit panel; `menuTarget` remembers the right-clicked row.
+- `context-menu.service.ts` + `context-menu.ts` — the panel right-click menus,
+  in HTML. Electrobun's native `showContextMenu` is a no-op on Linux, so it was
+  retired on ALL platforms rather than kept as a second code path (see
+  platform-notes.md). The service builds the item list (row / empty-space menu,
+  and the header column picker) and holds the open request as a signal; the
+  component renders it at the pointer with edge-flip clamping, arrow/Enter/Escape
+  keys, and close-on-blur/resize/scroll. Picked items are emitted verbatim as
+  `ctx-*` / `col:*` strings through `RpcService.emitMenuAction`, so they land in
+  the SAME `App.handleMenuAction` dispatcher the macOS app menubar uses — one
+  handler, two sources. The three shell actions the webview cannot perform
+  (reveal, open-with-default, copy path) go back to bun via the `shellItemAction`
+  RPC, which is all that remains of the old native menu.
 - `file-icons.ts` — extension → category map driving glyphs, colors
   (fixed `--icon-*` tokens, never accent-tinted) and the Kind column.
 - `platform.ts` — preload-global readers (`PLATFORM`, `APP_MODE`,
-  `EDITOR_CONTEXT`, `IS_MAC_CHROME`, `IS_WINDOWS`, `MOD_LABEL`/`ALT_LABEL`)
-  + `blockNativeContextMenu` (kills WebKit's Reload/Inspect menu outside
-  text fields).
+  `EDITOR_CONTEXT`, `IS_MAC`/`IS_MAC_CHROME`, `IS_WINDOWS`, `IS_LINUX`,
+  `MOD_LABEL`/`ALT_LABEL`) + `blockNativeContextMenu` (kills WebKit's
+  Reload/Inspect menu outside text fields — the app draws its own menu instead).
 - `updater.service.ts` — auto-updater state for the tab-bar pill. Signals
   `state` (`idle→available→downloading→ready`|`error`), `version`, `progress`.
   Subscribes the `updateStatus` push (backend `Updater.checkForUpdate` on