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>

src/bun/rpc-handlers.ts

@@ -1,7 +1,6 @@
 import { BrowserView, Updater, Utils } from "electrobun/bun";
 import type { AppRPC, EditorOpenContext } from "../shared/rpc";
 import { effectiveConcurrency, type AppSettings, type Result } from "../shared/types";
-import type { PanelContextMenu } from "./context-menu";
 import { readFileText, writeFileText, type EditManager } from "./edits";
 import { listLocalDrives } from "./fs/local";
 import type { SessionRegistry } from "./fs/registry";
@@ -24,7 +23,6 @@ export interface AppDeps {
 	store: ConnectionStore;
 	settings: SettingsStore;
 	edits: EditManager;
-	contextMenu: PanelContextMenu;
 	watcher: LocalDirWatcher;
 	/** Assigned after the rpc object exists (it needs rpc.send). */
 	events: {
@@ -122,7 +120,6 @@ export function createAppRPC({
 	store,
 	settings,
 	edits,
-	contextMenu,
 	watcher,
 	events,
 }: AppDeps) {
@@ -282,10 +279,12 @@ export function createAppRPC({
 						return {};
 					}),
 
-				// ---- context menu + local shell integration ----
-				showRowMenu: (params) =>
+				// ---- local shell integration (menu itself is HTML, see ui/) ----
+				shellItemAction: ({ action, path }) =>
 					wrap(async () => {
-						contextMenu.show(params);
+						if (action === "reveal") Utils.showItemInFolder(path);
+						else if (action === "open") Utils.openPath(path);
+						else Utils.clipboardWriteText(path);
 						return {};
 					}),
 
@@ -295,12 +294,6 @@ export function createAppRPC({
 						return {};
 					}),
 
-				showColumnsMenu: ({ visible }) =>
-					wrap(async () => {
-						contextMenu.showColumns(visible);
-						return {};
-					}),
-
 				// ---- editor windows ----
 				openEditor: (ctx) =>
 					wrap(async () => {