fix(update-dialog): draggable backdrop, selectable text, and localization

[RealZST]

Problem

The update dialog is mounted at the App root, outside <main>, so the app-shell's window-drag handler treated the dialog's text as draggable window chrome: text couldn't be selected, and dragging moved the whole window. The backdrop also dismissed the dialog when a drag happened to release over it. (Other dialogs render inside route pages — i.e. inside <main> — so they were already exempt.)

Change

Drag / selection

• Exclude [role='dialog'] from the window-drag gesture in app-shell, and dedupe the two identical guard blocks (onMouseDown / onDblClick) into one NO_WINDOW_GESTURE selector.
• Tag both update dialogs with role="dialog". Text inside the dialog is now selectable; the dimmed backdrop is left as plain chrome so pressing it still drags the window. Dismiss via the header ✕ / Later / Close.

Localization

• New update i18n namespace — the dialog title and buttons follow the app language.
• Bilingual release notes: localizeChangelog splits the changelog body on <!-- lang:en --> / <!-- lang:zh --> fences and shows the section for the active language, falling back to English, then to the whole body. Single-language notes without fences render unchanged.

Testing

• npm test (238) + cargo test --workspace green. Added 4 unit tests for localizeChangelog (language pick, regional-code normalization, fallback, no-fence passthrough).
• Verified in the desktop app: dialog text selects, dimmed area drags the window, double-click no longer maximizes; UI + changelog switch between English and Chinese with the app language.

Notes / follow-up

The dialogs keep the existing aria-modal="true" without a real focus-trap (consistent with the app's other hand-rolled modals). Adopting an accessible dialog primitive (Radix / native <dialog>) app-wide to add focus-trap + Esc + inert background is tracked as a separate a11y task.

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