feat(tui): UX hardening for approvals, thinking, tool steps, and errors

[jkyberneees]

Summary

Implements the top 5 findings from a holistic UI/UX review of the TUI. One cohesive commit; every change has regression tests.

• Approval flow: bare letters no longer decide — the panel is now an ↑↓-select + ⏎-confirm option list, so typing mid-prompt can't fire a decision. tab expands the full untruncated command (previously ellipsized to one line), esc is an explicit labelled deny, and transcript scrolling works while the panel is open.
• Thinking: ^T posts a note and shows a persistent ✳ think header badge; full reasoning is stored and ^E unfolds it on finalized turns (was permanently capped at 240 runes); the collapsed excerpt shows the head of the thought instead of the tail.
• Tool steps: expanded output preserves whitespace (diffs/JSON/code keep alignment); mouse toggling only fires on the step header row instead of any line below it; ^E shows a persistent ▼ details indicator; running steps get a chevron affordance.
• Feedback: send failures finalize the turn with an inline **Error:** (no more phantom empty turns); disconnect mid-turn finalizes with an **Interrupted:** marker (idempotent, resume-safe); /cancel reports idle/success/queue outcomes; the r retry hint only renders when it actually works; the disconnected-submit warning is sticky with correct advice.
• Narrow terminals: banner falls back to a one-line ⬡ bodek wordmark below the art width; model name truncates with ellipsis and the header is hard-clamped to its 2-row budget (viewport/mouse math can't drift); step-arg truncation floors lowered for tiny widths.

Test plan

• make fmt / make vet clean
• make lint — 0 issues
• make test — full -race suite green, all packages
• make cover — 98.9% internal coverage; all functions touched here at 100%
• New regression tests: approval key model, thinking expansion/badge, whitespace fidelity, click hit-testing, send-failure/disconnect finalization, cancel notes, retry hint, banner fallback, header clamp, tiny-width rendering