Add mouse click support to the TUI: click-to-select rows and picker options

[MJohnson459]

Add mouse click support to the TUI: click-to-select rows and picker options.

What changed

• ui::draw now returns a HitMap — a list of (Rect, Hit) built during the
  draw, where Hit is CockpitRow/TaskRow/ProjectRow/ViewerRow/
  PickerOption and the index is the one the key handlers already count in.
  Each list's rects come from the scroll offset ratatui computed while
  rendering it, so a scrolled list maps a visible line to the item on it.
  Key handlers stay geometry-free, as DETAIL_PAGE_STEP requires.
• App::on_mouse(col, row, &hitmap) clears status like on_key does and
  moves the same selection fields j/k move. A click never fires the row's
  action. In a picker, a click moves the cursor and a second click on the
  option already under it sends Enter through the existing key handler, so
  confirmation runs the unchanged transition path.
• A modal clears the map the screen behind it contributed, so only the popup's
  options are targets: clicks outside are ignored, not a dismiss, and
  text-entry modes (Prompt, LinkPr, AddProject, ViewerForm, ConfirmPr) and the
  Detail popup offer no targets at all.
• main.rs gains init_terminal/restore_terminal helpers that pair
  crossterm mouse capture with ratatui's init/restore at all four sites
  (startup/shutdown plus the $EDITOR, attach and plan round-trips), and a
  panic hook that disables reporting on the way out of a panic.
• DESIGN.md §9 gains a sentence naming the mouse as a secondary input.

Scope note: the document picker (Mode::DocPicker, added after this task was
written) is included with the other pickers — a click selects, a second click
link/unlinks, exactly as ⏎ does there.

Verification

• cargo test --workspace (559 tests) and
  cargo clippy --workspace --all-targets -- -D warnings are clean.
• Six new tests in ui.rs render into a TestBackend, capture the hit-map,
  and locate rows by the text drawn on them rather than by hard-coded
  coordinates: cockpit queue + running strip, dead zones (header, detail pane,
  status line), the tasks browser scrolled past its first page, the projects
  and Config viewer lists, picker select-then-confirm (ready -> park -> parked),
  and clicks outside a popup / in a text prompt. Mutation-checked the scrolled
  case by ignoring the offset — it fails, as it should.
• Drove the real binary under tmux against a scratch DB: a click moved the
  cockpit selection and the detail pane followed; two clicks on "park ->
  parked" in the transition picker parked the task; a click on the detail pane
  did nothing; a click selected the right row in the task browser (which
  orders by score, not id); clicks still worked after an $EDITOR round-trip.
• Captured the raw pty stream: mouse reporting is enabled once at startup and
  disabled before leaving the alternate screen on quit.

Branch tui-mouse-clicks, committed locally, not pushed.