Track Fable 5 weekly usage in place of the Sonnet-specific limit

[asirghi-cera]

What I'm seeing

ClaudeMeter version: 1.4.0
macOS: 15.x
Plan: Max

The Sonnet meter never moves for me. On claude.ai under Settings → Usage there's no
separate Sonnet limit anymore — the model-specific bar is Fable 5, which has its own
weekly cap (up to 50% of the plan's weekly limit on Max and premium Team/Enterprise
seats).

So "Show Sonnet Usage" is tracking a limit my account doesn't have, and the one
model-specific limit I do want to watch isn't shown anywhere.

What I'd like

Show Fable 5 weekly usage where Sonnet usage sits today — same popover row, same
notification thresholds, same menu bar indicator.

Better still, stop naming the model in code. If the usage response carries whichever
model-specific limits apply to the account, render one row per limit and label it from
the response. The next model swap then becomes a data change instead of a release.

Places this touches

• Settings → General: the "Show Sonnet Usage" toggle and its subtitle
• Menu bar popover and the secondary indicator
• ~/.claudemeter/usage.json: the sonnet_usage key. Adding fable_usage (or a
  generic model_usage object) next to it keeps existing statusline scripts working.
  Removing sonnet_usage outright would break them.
• README feature list and the docs/menubar-sonnet.png screenshot

One thing worth handling

Fable counts against plan limits on Max and premium seats only. On Pro and standard
Team/Enterprise seats it runs on usage credits, so there may be no Fable bar at all.
Treating that as "no data" rather than 0% would avoid a misleading green meter.

Reference: https://support.claude.com/en/articles/15424964-claude-fable-5-on-your-plan