[Waddas]

What does this PR do?

Polishes the widget and settings with clearer typography, softer surfaces, consistent controls, and visible keyboard focus while preserving the existing layouts and mascot behaviour.

• Adds four saved themes: Charcoal (default), Midnight, Paper, and Mist. Theme previews make them easy to compare, and both windows and the graph update immediately when switching.
• Adds compact previews for all six widget layouts in a two-row grid.
• Replaces the graph's full-height reset line with a small tick and softens the dotted forecast. The ceiling stays neutral until current or projected usage reaches the existing 80% warning threshold.
• Reveals the selected 5h/7d/model label after right-clicking the graph; the time and percentage readout returns when the cursor moves.
• Defaults older settings files to Charcoal without resetting other preferences.

Related issue

None.

Checklist

• task check passes locally (type-check, Clippy lint, 77 frontend tests and 92 Rust tests)
• The PR title follows Conventional Commits
• Docs/comments updated for the new behaviour
• Verified on macOS using browser rendering and interaction checks with mocked Tauri state

Screenshots / notes

npm run build also passes. Browser checks covered all four themes, compact layouts, keyboard selection, theme changes across both windows, and normal, near-limit, and missing-data graph states. Theme persistence and migration are covered by Rust tests. Native end-to-end tests were not run; the existing suite is Windows-only.