[4/7] TUI rendering & navigation for N-level headers

[shouze]

Context

The interactive TUI renders exactly one section-header style today (src/render.ts, around the section-row block): a magenta ── label line, with pick-mode bar support and a multi-team hint detected via label.includes(" + "). Row in src/types.ts has no depth field. Header-row mouse hit-testing lives in src/render/mouse-hit.ts.

Part of EPIC #125. Depends on #177.

Solution

• Add a level (or reuse tree depth) to Row so buildRows (src/render/rows.ts) can emit one row per header at any depth.
• Render depth-dependent indentation/style in src/render.ts (increasing indent per level, consistent color family), preserving pick-mode bar and multi-team hint behavior at whichever depth is a combined section.
• Adjust hit-testing in src/render/mouse-hit.ts and src/render/layout-constants.ts for header rows spanning multiple depths.
• Verify fold/unfold, filter mode, and scrolling still behave correctly with nested headers.

Acceptance Criteria

• A 2+ level hierarchy renders visually distinct nested header rows in the TUI, each independently navigable with ↑/↓.
• Pick mode (p) and the multi-team hint still work correctly on a combined section at any depth.
• Mouse clicks on any header row at any depth hit-test to the correct row/zone.
• Fold/unfold and filter mode work unchanged with nested headers.

Definition of Done

• Updated tests in src/render.test.ts, src/render/mouse-hit.test.ts, and rows-related tests.
• Manual TUI verification checklist completed (navigation through nested headers, fold/unfold, filter mode, mouse clicks) and noted in the PR description.
• bun test, bun run lint, bun run format:check, bun run knip, bun run build.ts all green.
• Depends on [1/7] Data model & core hierarchical grouping algorithm #177 being merged first.