Add Fable weekly quota tracking

[ucodia]

Summary

Adds tracking for the Claude Fable 5 weekly usage limit, shown as a new Weekly Fable card in the menu bar popover.

• Parses the Fable quota from both API response shapes: the dedicated seven_day_fable field, and the weekly_scoped entry in the limits array that migrated accounts receive (where the old seven_day_* keys are null and the array reports percent with the model display name in scope).
• New Show Fable Usage toggle in Settings (show_fable_usage, off by default, decodes old saved settings safely).
• withFable demo mode for screenshots.
• The popover now sizes itself to its content instead of a fixed 320×460 frame, so a third (or fourth) card fits without scrolling.
• Test runs no longer read the real keychain: the app skips bootstrap when launched as an XCTest host, which previously triggered repeated keychain password prompts on unsigned builds.

Screenshot

Testing

• New unit tests cover both API shapes, including a raw-JSON fixture modeled on a real migrated-account response, plus settings decoding defaults.
• Full suite passes (53 tests); the 8 MenuBarIconSnapshotTests failures are pre-existing on a clean checkout (reference PNGs recorded on a different macOS build) and unrelated.
• Verified end-to-end against the live API: the card renders with real Fable utilization (see screenshot).

Generated with AI assistance

[mpecan]

Hi @ucodia #32 -> this already does the same, but makes is track "any" models provided in the response by Anthropic.

[ucodia]

Closing in favor of #32