feat: add fit-to-window graph layout (#1792)

* feat: add fit-to-window graph layout

Add persisted fit-to-window and margin settings for Usage charts. Resize visible charts across the available viewport, remove hidden sidebar spacing in fullscreen, and group the related controls under Graph Size.

Validated with frontend and Rust tests, Biome, rustfmt, Clippy, and the production frontend build.

* feat: roll back failed graph setting writes

Restore the prior in-memory graph preference when persisting the new value fails, keeping runtime state aligned with settings.json. Add failure-path tests for both graph layout setters.