feat(settings): add a display-size selector for the WhatsApp webview

[maerlin]

Problem

WhatsApp Web sizes its layout in px and has no scaling control of its own, so the chat list occupies the same width whatever the display. On a large high-DPI screen that leaves the conversation pane cramped while the chat list takes a disproportionate share of the window, and nothing inside whatRust could change it.

Change

A Display size selector under Settings → Preferences: Smaller (0.75), Small (0.85), Default (1.0), Big (1.15).

• Stored as a zoom factor in settings.json, defaulting to 1.0, so existing files keep today's behaviour with no migration.
• Applied through the webview's own page zoom (WebviewWindow::set_zoom), not a font-size override. A font-size override moves almost nothing on a page that sizes in px; scaling the page is what moves the chat list and the conversation together.
• Set when an account window is built, and re-applied to every open account window when the setting changes, so a new size takes effect without a restart.
• Saves on change rather than waiting for Save, since a size you have not seen is a guess.

Robustness:

• zoom is clamped to 0.5–2.0 on load and on save, and a non-finite value falls back to 1.0, so a hand-edited settings.json cannot leave the app at a size you cannot read your way back out of.
• The settings UI shows the option nearest the stored value rather than assuming it is one of the four presets.
• Page zoom needs macOS 11 or later (Tauri's documented constraint); below that the call is a no-op and the window keeps the site's own sizing.

The select gets appearance: none plus an inline-SVG chevron: a native popup button carries fixed control metrics and ignores most of the .field padding, so it rendered visibly shorter than the text inputs beside it.

Testing

• 4 new unit tests in settings.rs covering the presets, clamping, non-finite input, and an older settings.json with no zoom key. Full suite: 70 pass. cargo fmt --check clean.
• Built a release bundle and ran it on macOS 15 (Apple Silicon) against a live account: switching size applies immediately across open account windows and survives a restart.
• Not exercised on Linux or Windows beyond CI's compile; set_zoom is Tauri's own cross-platform API and there is no platform-specific code in this change.