fix(tui): address PR review feedback

Fixes from OutThisLife review:
1. Restore Linux Alt+Enter newline: textInput.tsx now uses
   k.shift || (isMac ? isActionMod(k) : k.meta) so Alt+Enter
   inserts a newline on Linux (was broken by isMac guard).
2. Fix image.attach response type: useComposerState.ts now uses
   ImageAttachResponse (which already has remainder) instead of
   InputDetectDropResponse with intersection.
3. Expand looksLikeDroppedPath test coverage with edge cases for
   image extensions, file:// URIs, spaces, empty input, and
   non-file URLs.
4. Make terminalParity.test.ts hermetic: terminalParityHints() now
   accepts optional fileOps/homeDir and passes them through to
   shouldPromptForTerminalSetup(), so tests inject mock readFile
   instead of hitting the real filesystem.

Fixes from Copilot inline review:
5. Remove unused options.now parameter from configureTerminalKeybindings.
6. Replace naive stripJsonComments (full-line // only) with a proper
   JSONC stripper that handles inline // comments, block comments,
   trailing commas, and preserves comment-like sequences in strings.
7. Move backupFile() call from immediately after read to right before
   write - backups are only created when changes will actually be
   written, not on every /terminal-setup invocation.

ui-tui/src/components/textInput.tsx

@@ -573,7 +573,7 @@ export function TextInput({
       }
 
       if (k.return) {
-        k.shift || (isMac && isActionMod(k))
+        k.shift || (isMac ? isActionMod(k) : k.meta)
           ? commit(ins(vRef.current, curRef.current, '\n'), curRef.current + 1)
           : cbSubmit.current?.(vRef.current)