feat: Codex and Claude Code usage stats — Settings → Usage page and composer context popover

[Tryanks]

Summary

Adds account usage / rate-limit visibility for Codex and Claude Code, following t3code's approach (ask the provider CLIs; never touch their OAuth tokens):

• Data: codex app-server → account/rateLimits/read; claude -p --input-format stream-json → get_usage control request. Normalized into presentation-free tcode_core::usage::{ProviderUsage, UsageWindow} (5h / weekly / model-scoped windows, plan, reset time). Fetched on startup, after each completed turn (throttled to 2 min), and on demand via Command::RefreshProviderUsage; flows through the existing ProvidersStatus snapshot.
• Settings → Usage: one card per enabled Codex / Claude Code profile with plan chip, per-window label, percent, "Resets in …", and a bar (blue < 75%, amber 75–90%, red ≥ 90%). Refresh button + "Updated … ago".
• Composer: the context-window popover now shows a Usage block (plan + the same windows) for the active session's profile.
• Rows are rendered exactly as reported: a Codex Pro account has no 5h window and shows only Weekly; Claude Max shows 5h, Weekly, and the separate Weekly · Fable window.
• Locales: en + zh-CN.

Verification

• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace — all green.
• Live fetch against signed-in Codex (Pro) and Claude Code (Max) on macOS: ignored test provider_usage::tests::live_provider_usage passes.
• Eyes-on in a throwaway profile (TCODE_DATA_DIR): Settings → Usage and both provider popovers render the expected rows; no layout defects.

Not exercised: amber/red bar states (usage stayed below 75% during verification), Linux/Windows.