feat(tui): add balance status item

[MoriTang]

Summary

支持底部状态栏自定义配置（[statusline]），显示 API 余额等实时信息 #1551

Adds a DeepSeek account balance chip to the footer status bar. The balance
is fetched from GET /user/balance once per turn completion and displayed
alongside mode, model, and session cost.

Non-DeepSeek providers never see the Balance toggle — the picker and default
footer both hide it behind StatusItem::is_available_for(provider).

Changes

Area	Description
StatusItem::Balance	New variant with key/label/description/hint
pricing.rs	BalanceResponse / BalanceInfo deserialization structs
tui/ui.rs	fetch_deepseek_balance() — async HTTP fetch using the configured API key
tui/footer_ui.rs	footer_balance_spans() — formats balance with currency-appropriate precision
tui/widgets/footer.rs	FooterProps.balance field, left-cluster rendering in the footer widget
tui/views/status_picker.rs	StatusPickerView::new(active, provider) filters rows by is_available_for
config.rs	default_footer() excludes Balance (provider-specific); it must be opted in via /statusline

Provider gating

• Fetch triggers only for Deepseek / DeepseekCN providers
• StatusItem::is_available_for() returns false for Balance on all other providers
• The /statusline picker filters unavailable items so non-DeepSeek users never see the toggle

Tests

New tests in pricing.rs, ui/tests.rs, config.rs, and status_picker.rs:

• BalanceResponse JSON deserialization (3 cases)
• total_balance_f64() parsing — valid / empty / invalid (3 cases)
• footer_balance_spans() — empty cell, zero, CNY, USD, large-amount rounding, unknown currency (6 cases)
• render_footer_from with/without Balance in items (2 cases)
• is_available_for across all known providers (1 case)
• Picker row filtering for non-DeepSeek provider (1 case)

No regressions: 191 tests in the impacted modules pass.

Testing

• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes

[gemini-code-assist]

Code Review

This pull request introduces a DeepSeek account balance display in the TUI footer. Key changes include the implementation of background fetching for balance information after each turn completion, localized strings for the balance prefix, and updates to the footer rendering logic to include the new balance chip. Additionally, the status picker was enhanced with provider-specific filtering and scrolling capabilities. Review feedback suggests adding a timeout to the background HTTP client to prevent stalled tasks, displaying the balance even when it is zero to notify users of empty accounts, and improving the currency symbol mapping to handle future currency additions more robustly.

[Hmbown]

Independent review:

Reviewed the feature against #2257 with a real merge sim (#1970 has 264-file conflicts against current main; #2257 is rebased cleanly).

• Implementation quality is sound. BALANCE_CLIENT is a LazyLock<reqwest::Client> with a 10s timeout (good — your recent fix). fetch_deepseek_balance is fire-and-forget via tokio::spawn, writes through Arc<Mutex<Option<BalanceInfo>>>. Errors return None so stale balance is preserved.
• Provider gating is correct. StatusItem::is_available_for(Deepseek|DeepseekCN) → true, others false. Picker filters before display, default footer excludes Balance (opt-in via /statusline).
• Test coverage is genuinely thorough. 6 balance unit tests + footer render tests + picker filter test + is_available_for matrix. Better than typical PRs at this surface area.
• Frequency is reasonable — once per TurnComplete plus startup/SwitchProvider, not polled.
• What feat: add account balance status bar item (carries #1970) #2257 added on top of feat(tui): add balance status item #1970: rebase onto main + 3 Clippy fixes + zero-balance hides chip + Balance positioned after Cost. Nothing unique remains in feat(tui): add balance status item #1970 that feat: add account balance status bar item (carries #1970) #2257 omits.
• What feat(tui): add balance status item #1970 still uniquely shows: zero balance was DISPLAYED (per gemini-code-assist feedback to notify empty accounts); feat: add account balance status bar item (carries #1970) #2257 reverted that — worth confirming which UX you actually want.

v0.8.48 (#2256) compatibility: PR #2256 deletes tui-core and restructures workspace crates (5342 deletions) — both #1970 and #2257 will conflict heavily on crates/tui/src/**, but #2257's rebase is recoverable; #1970's is not. Superseded by #2257: yes — recommend closing #1970 in favor of #2257.

[Hmbown]

@MoriTang — appreciation for the original balance feature. After the v0.8.45 → v0.8.46 series, this branch is now 264 files behind main and the rebase has become impractical. PR #2257 (HUQIANTAO) explicitly carries this work — rebased to v0.8.47, fixed 3 clippy warnings, reordered Balance after Cost, and your 6 balance unit tests are still there. So your design + tests are alive in #2257.

One specific call to confirm: #2257 currently suppresses the balance display when balance is zero (reverted on reviewer feedback). Your original #1970 shows zero. If you have a strong opinion on which UX is right — particularly for free-tier users who'd want to see "0 / out of credits" rather than silence — flag it on #2257 and we'll honor it before merge.

Otherwise: thanks for showing up first. Your stamp is on this feature regardless of which PR ships.

[MoriTang]

@MoriTang — appreciation for the original balance feature. After the v0.8.45 → v0.8.46 series, this branch is now 264 files behind main and the rebase has become impractical. PR #2257 (HUQIANTAO) explicitly carries this work — rebased to v0.8.47, fixed 3 clippy warnings, reordered Balance after Cost, and your 6 balance unit tests are still there. So your design + tests are alive in #2257.

One specific call to confirm: #2257 currently suppresses the balance display when balance is zero (reverted on reviewer feedback). Your original #1970 shows zero. If you have a strong opinion on which UX is right — particularly for free-tier users who'd want to see "0 / out of credits" rather than silence — flag it on #2257 and we'll honor it before merge.

Otherwise: thanks for showing up first. Your stamp is on this feature regardless of which PR ships.

I think hide zero is OK. Thanks for carried my commits.