feat: add MiniMax token-plan provider

[lamchun1110]

Adds a MiniMax provider so OpenQuota can show MiniMax token-plan usage.

What

• New provider at providers/minimax/ (modeled on providers/zai/): client, mapper, auth, mod.
• Reports two quotas using the existing naming convention:
  • Session — the per-interval remaining percent (current_interval_remaining_percent)
  • Weekly — the weekly remaining percent (current_weekly_remaining_percent)
  • Uses the general model entry, falling back to the first model if general is absent.
• Source: GET https://www.minimax.io/v1/token_plan/remains, Bearer API key.
• Success is gated on base_resp.status_code == 0; a "no token plan" message maps to a Permission error (like Z.ai's NoCodingPlan).
• Key stored in the OS keychain (or MINIMAX_API_KEY / ~/.config/openquota/minimax.json), wrapped in Zeroizing, never logged.
• Registered in the runtime list (lib.rs), declared in providers/mod.rs, covered by the provider-registry contract and the provider-links test.
• Unit tests: captured payload mapping, model fallback, error classification, missing/invalid/rate-limited keys.

Verification

• `cargo test -p openquota minimax` — 7/7 passing
• `pnpm verify:contracts` — 11 runtimes; `pnpm check` — 0 errors

Caveat

The `/v1/token_plan/remains` endpoint is undocumented (discovered against the live API). Field paths may need adjustment if MiniMax changes the response.

Contributed under the repo's MIT license. Built/tested on Linux and macOS.

[lamchun1110]

Superseded by #38, which combines Kimi and MiniMax into a single PR. The two were split across branches that touch the same shared files (registry, contract test, icon map) and would conflict on merge, and #38 also includes the status-bar icon fix so the logos render in the menu bar. Closing in favor of #38.