Fix Gemini paid-tier plan display

[Finesssee]

Summary

• Ports upstream steipete/CodexBar v0.42.0 commit 6517892b1 / PR Fix Gemini Plus plan display from loadCodeAssist paidTier steipete/CodexBar#1980 plan resolution into the Gemini provider.
• Calls loadCodeAssist and prefers paidTier.name over valid generic Free, Workspace, or Paid fallbacks.
• Keeps the resolved Gemini label in the existing UsageSnapshot.login_method, so the existing Tauri serialization and UI consumers receive it without a frontend branch.

Tests

• Paid tier wins over free/workspace and standard/paid fallbacks.
• Missing paid tier preserves Workspace, Free, and Paid fallback resolution.
• Malformed optional paidTier data does not erase a valid currentTier fallback.
• Invalid status does not synthesize a generic plan.
• CLI output preserves Gemini Code Assist in Google One AI Pro acronym casing.

Already present / not applicable

• Existing Gemini OAuth, CLI source mode, quota parsing, identity isolation, snapshot bridge, and UI consumers remain unchanged.
• No frontend changes, new dependencies, or unrelated provider work.

Validation

• Focused post-fix regression: cargo test --manifest-path rust/Cargo.toml providers::gemini::api::tests::malformed_paid_tier_preserves_current_tier_fallback -- --exact --nocapture (passed)
• cargo fmt --manifest-path rust/Cargo.toml -- --check (passed)
• cargo fmt --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml -- --check (passed)
• git diff --check (passed)
• Full shared/Tauri Rust tests and both clippy commands passed on the initial implementation commit; post-fix reruns were attempted but blocked by pre-existing concurrent Cargo package/build locks in the shared worktree.