[0.64.1] Add preferred display currency and TRY support

[Finesssee]

Port the v0.64.1 preferred display currency behavior, including TRY. A persisted preference controls presentation in Usage & Spend, tray labels, FloatBar, provider cost views, Claude Swap, and share PNGs. Conversion uses cached live rates with bundled fallback rates; copied and saved JSON exports retain source values.

The CI follow-up corrects Rust formatting and the Tauri async command return type. It also preserves non-currency unit labels such as Credits when conversion is unavailable, as required by an existing regression test.

Validation:

• cargo fmt --all --check and git diff --check: passed.
• cargo clippy --workspace --all-targets -- -D warnings: passed on Windows.
• cargo test --workspace currency -- --test-threads=4: 14 passed across backend and Tauri.
• Focused Vitest checks for currency, usageSpendSharing, GeneralTab, and ClaudeSwapAccountsSection: 40 passed.
• pnpm --dir apps/desktop-tauri build: locale check, TypeScript, and Vite build passed.
• Independent thermo review of the two corrective changes: approved.
• Merged the shared CI test fixes from Stabilize tray layout and Claude login tests in CI #607 into this branch; the full hosted check is running for the updated head.

This remains a draft pending a fresh Windows desktop build and native UI proof of the complete feature.

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[Finesssee]

CI and review follow-up

Fixed the rustfmt failure, then the next hosted failure in the Tauri command macro: async commands borrowing State must return Result. The command now returns Ok(CurrencyRatesSnapshot), preserving the successful invoke payload.

The focused frontend run also caught an existing assertion failure (4.25 Credits was rendered as 4.25 CREDITS). The formatter now preserves non-currency labels while normalizing currency codes.

Verification: workspace clippy with warnings denied passed; 14 Rust currency tests and 40 focused frontend tests passed; the frontend production build and formatting checks passed. The two corrective changes received an independent thermo review with no blocker. These checks do not substitute for the complete feature's pending native Windows UI proof, so the PR remains draft.

The branch also contains the merged #607 CI test corrections. The full CircleCI gate must pass on the updated head.