feat(tui): surface pay-per-use credits in the account list and details

[xjoker]

Problem / intended behavior

The TUI never gave pay-per-use credits a stable home, even though the data (UsageInfo.credits_balance / unlimited_credits) is already parsed, cached, and shown in the CLI (print_usage_line) and JSON output. Two gaps:

1. The account list table had no credits column.
2. The Enter → Account details popup only showed credits as a conditional dim line inside usage_meta, and only when a balance existed — so subscription accounts (e.g. Team, which have no credits_balance) had no credits slot at all.

This adds a Credits column to the list and a dedicated Credits field to the details popup.

Implementation boundary

src/tui/ui.rs, src/tui/menu.rs, src/tui/app.rs — rendering only, no data-flow changes.

• List column (ui.rs): new Credits header + per-row cell after Cards. Helpers credits_table_text / credits_table_color:
  • Text mirrors the CLI wording: unlimited / $x.xx / -- (the last for accounts without the credits system, consistent with the CLI hiding $0.00).
  • Color reuses the color::credits low-balance thresholds (≥10 green, ≥2 yellow, else red) mapped to the TUI palette. It intentionally does not call color::credits, which emits ANSI escapes that would corrupt ratatui output.
  • Idle/Error → --, Loading → ..., matching neighboring columns.
  • Widened the fixed-column budget in table_text_widths 64 → 75 for the extra width-10 column + gap.
• Details popup (menu.rs): an always-present Credits line under Quota pools, styled like the Reset cards field, showing $x.xx / unlimited / not available. Color shared via credits_table_color (now pub(super)).
• Dedup (app.rs): removed the old conditional credits entry from usage_meta so the balance isn't shown twice.

Out of scope: a credits line in the bottom render_usage_gauges detail panel, and a credits sort mode.

Tests

• credits_column_text_and_color_track_the_balance_state — wording + green/yellow/red/dim thresholds.
• account_table_renders_the_credits_column — renders the real table via TestBackend, asserts the Credits header and a $15.50 balance.
• account_details_show_a_dedicated_credits_field — renders the popup, asserts the Credits field with $15.50 (green) and the not available placeholder for no-credits accounts.
• Verified existing table_text_widths tests still hold under the 64→75 change.

Verification (Linux)

cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all — all pass, 0 failed.

Live run with a real logged-in account (Team plan, no pay-per-use credits) using the installed release binary:

List column:

│   Alias Emai Plan Status 5h     7d     5h Reset     7d Reset     Cards   Credits │
│*  oai07 oai0 Team OK     100%   100%   19:20        09-01 14:20  1       --      │

Enter → Account details (new field under Quota pools):

 Quota pools
 Main
 5h  ░░┃░░░░░░░░░░░░░░░░░░░  100% left · reset 4h35m (19:20)
 7d  ┃░░░░░░░░░░░░░░░░░░░░░  100% left · reset 6d23h (09-01 14:20)
 Credits  not available
 Reset cards  1 available

Team accounts correctly read not available (no credits_balance); pay-per-use accounts render $x.xx / unlimited (covered by the render tests).

Unverified

• macOS / Windows rendering (relies on the CI matrix).
• Live $x.xx balance shown only via TestBackend/unit fixtures — the available real account is a Team plan without credits.