[Bug]: Desktop (Tauri) mode — Ctrl +/- / Ctrl + wheel zoom does not work on Linux

## Description
When running QwenPaw in desktop mode (`qwenpaw desktop`, Tauri shell) on Linux,
the standard in-app zoom shortcuts `Ctrl +` / `Ctrl -` and `Ctrl + mouse wheel`
have no effect. The font size / UI scale cannot be adjusted from within the app.

## Environment
- OS: Ubuntu 24.04 (Linux x86_64)
- QwenPaw mode: Desktop (`qwenpaw desktop`) — Tauri shell over the Web UI
- QwenPaw version: 2.0.0.post3
- Display server: Wayland

## Steps to Reproduce
1. Launch `qwenpaw desktop` on Linux.
2. Wait for the Tauri window to open.
3. Press `Ctrl +` or `Ctrl -` (or hold `Ctrl` and scroll the mouse wheel).
4. Observe that the UI text / layout does not scale.

## Expected Behavior
- `Ctrl +` / `Ctrl -` should zoom the in-app UI (or at least be captured).
- Ideally, a settings entry for font size / UI scale, or respect for the OS
display scaling, would be available.

## Actual Behavior
- No visible change to font size or layout.
- Shortcuts appear to be ignored by the Tauri WebView.

## Additional Context
- The official desktop docs (desktop.zh.md) only cover Windows and macOS and
mention no font-size setting; Linux desktop is not documented separately.
- This may be a Tauri WebView shortcut-handling issue rather than app logic.