Merge main into feat/7css-skin + give the HTML context menu an Aero skin

main retired electrobun's native context menu (a documented no-op on Linux) in
favour of an HTML one on every platform. Merging that here needs the menu to
look right under the Win7 skin as well.

Conflicts, all resolved by keeping both sides:
- src/bun/index.ts — createAppRPC keeps the branch's mainWindow getter, drops
  main's removed contextMenu dep.
- ui/src/app/app.ts — imports both TitleBar and ContextMenu.
- ui/src/app/platform.ts — kept the branch's version (its IS_MAC_CHROME accounts
  for the frameless win7 chrome, and MOD_LABEL correctly keys on the real
  platform rather than the chrome), plus exporting IS_MAC and adding IS_LINUX
  for the menu's per-OS shell wording.

Aero menu (branch-only): 7.css has no menu component, so it is hand-authored
across both layers — the row <button>s shed their push-button slab in `skin`
(revert-layer only reaches `utilities` from there), and the square-cornered
slab, hairline border and gradient selection band are stated in `skinfix`.
Built strictly from existing --w7-* vars, never literal hex, so the
hand-written dark Win7 palette recolours it for free. Hover and the keyboard
cursor share one band (the component marks the cursor with `bg-accent`, which
is overridden alongside :hover) as a real Win7 menu does.

Also answers a question the merge raised: frameless windows DO work on Linux.
The GTK wrapper honours titleBarStyle "hidden" — it calls
gtk_window_set_decorated(false) and logs "GTK: Created window without
decorations (custom titlebar)". Verified on GNOME/Wayland and documented in
architecture.md.

Verified: backend `tsc --noEmit` clean, `bun run build:ui` clean (492.52 kB
initial, still under the 500 kB budget), app launches with the skin active,
frameless, no console errors and no GBM failures.

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

src/bun/rpc-handlers.ts

@@ -1,7 +1,6 @@
 import { BrowserView, type BrowserWindow, 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;
 	/**
 	 * The main BrowserWindow, for the win7 skin's custom title-bar controls.
@@ -175,7 +173,6 @@ export function createAppRPC({
 	store,
 	settings,
 	edits,
-	contextMenu,
 	watcher,
 	mainWindow,
 	events,
@@ -339,10 +336,12 @@ 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 {};
 					}),
 
@@ -352,12 +351,6 @@ export function createAppRPC({
 						return {};
 					}),
 
-				showColumnsMenu: ({ visible }) =>
-					wrap(async () => {
-						contextMenu.showColumns(visible);
-						return {};
-					}),
-
 				// ---- editor windows ----
 				openEditor: (ctx) =>
 					wrap(async () => {