[fylorn]

Summary

Adds an interface size setting that scales the whole UI through the webview zoom (Tauri's set_zoom: WKWebView pageZoom, WebView2 ZoomFactor, WebKitGTK zoom-level). Layout, icons and spacing grow together; no CSS changes.

• Settings > General > Interface size, right under Appearance: 90% / 100% / 110% / 125% / 150% (segmented control), default 100%, applied on click like the other rows.
• Shortcuts: Cmd/Ctrl+= (also +) zooms in one step, Cmd/Ctrl+- zooms out, Cmd/Ctrl+0 resets to 100%. Registered in palette/keys.tsx, shown in the shortcut sheet, and searchable in the command palette. The next step is computed on the Rust side, so a held key does not read the same step several times. Tauri's zoom_hotkeys_enabled stays off.
• Storage: zoom in app.json (Prefs::zoom, stored as a percentage, read in from_fields; values outside the steps fall back to 100%). It is not in the client_prefs sync list, because the right size depends on each computer's screen. A test covers this.
• No 100% flash on cold start: the main window is zoomed in Rust right after it is built, before the page loads.
• Window sizes: the main window's default size and minimum size scale with the zoom, so its CSS width matches 100% and does not drop below the 768/640 breakpoints. Both are clamped to the work area of the screen (150% needs 1230×840 pt, which does not fit vertically on a 1080p screen at 125% system scaling). Changing the zoom resets the minimum size, and the window grows if it is smaller. When the clamped size fills one side of the screen, the window is centered.
• Small windows: the update window and the connection picker follow the zoom too. window::fit multiplies the measured CSS size by the zoom factor; the native title bar is not scaled. When the zoom changes while one is open, its ResizeObserver fires because the viewport width changes, and it refits.

No new platform-specific branches: everything goes through Tauri's cross-platform set_zoom, set_min_size and monitor APIs.

Test plan

• tsc -b --noEmit, vitest run (new test: shortcut key mapping, and the UI steps match STEPS in zoom.rs)
• cargo clippy --all-targets -- -D warnings, cargo test (new tests: serialization and steps, minimum-size scaling and clamping, prefs round trip and invalid values, not synced)
• Full-app preview with the commands mocked: the settings row, the shortcut sheet entries, palette search, and the shortcuts change the selected step; dashboard rendered as at 125% (880×576 CSS px for a 1100×720 window) without horizontal overflow

Closes #355

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