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/shared/rpc.ts

@@ -1,6 +1,5 @@
 import type {
 	AppSettings,
-	ColumnKey,
 	ConnectionConfig,
 	ConnectionsFile,
 	EntryType,
@@ -126,20 +125,18 @@ export type AppRPC = {
 				response: Result<{ names: string[] }>;
 			};
 			clearEditLeftovers: { params: Record<string, never>; response: Result };
-			// context menu + local shell integration
-			showRowMenu: {
-				params: {
-					path: string;
-					name: string;
-					type: EntryType;
-					isLocal: boolean;
-					hasEntry: boolean;
-				};
+			// local shell integration
+			/**
+			 * Shell actions the context menu triggers on a LOCAL path. The menu
+			 * itself is HTML (ui/src/app/context-menu.ts) — electrobun's native
+			 * context menu is a no-op on Linux — but these three still need the
+			 * bun side's Utils.
+			 */
+			shellItemAction: {
+				params: { action: "reveal" | "open" | "copyPath"; path: string };
 				response: Result;
 			};
 			moveToTrash: { params: { path: string }; response: Result };
-			/** Native column-picker menu (right-click on the list headers). */
-			showColumnsMenu: { params: { visible: ColumnKey[] }; response: Result };
 			/** Native About box (name, Build n, commit, engine versions). */
 			showAbout: { params: Record<string, never>; response: Result };
 			/** Windows drive roots for the local panel ("C:\", …); empty elsewhere. */