Phase 5.10: Scale the whole app with persisted controls

[danielmrdev]

What to build

Add a curated App scale Preference for the complete interface. A writer can enlarge or reduce the titlebar, Writing surface, panels, and Settings together and recover the same scale after restart.

Acceptance criteria

• General Settings exposes App scale from 80% through 200% in 10% steps with a 100% default.
• Ctrl/Cmd plus, minus, and zero change or reset the same persisted Preference.
• Scale applies live to the complete webview and composes with operating-system DPI scaling.
• Scaling does not mutate Font size, Document content, caret intent, or saved File state.
• The full supported range keeps controls operable, focus visible, and modal content reachable at the Linux gate window size.

Blocked by

• Phase 5.9: Switch the interface between English and Spanish #45

Workspace

• Mode: parallel lane
• Branch: feat/tauri-2-replatform-46
• Worktree: /home/daniel/Projects/lettera/.worktrees/feat-tauri-2-replatform-46
• Integrates into: feat/tauri-2-replatform
• Remote policy: local only — never push
• Completion: when unblocked, create the lane from the latest integration commit; validate it, merge it locally into integration with approval, validate integration, remove the lane worktree, and delete the local lane branch

[danielmrdev]

Frozen by the approved UI stack decision gate in #49. Do not implement this direct-migration ticket while the Floem/Tauri bake-off is open. Native dependency #54 records the live gate. After the blind decision, this ticket will be reviewed and respecified only if Tauri wins; if Floem wins, it will be closed as replaced.