feat(layout): fullscreen + interactive pane resizing

Add a window-management layer so the chat, roster and sandbox-terminal
panes can be resized live and fullscreened, with named presets for
recall.

- New layout.rs: single source of truth for the body split (pty_pct,
  roster_width) and a Zoom state (Normal/Term/Chat), persisted to
  layouts/<slug>.toml like themes. Both ui::draw and app::sbx_dims read
  from it, so resizing just mutates state and clears announced_dims —
  the per-tick loop re-syncs and broadcasts the new PTY grid.
- F4 cycles terminal/chat fullscreen (F-keys aren't forwarded to the
  shell, so nothing is stolen from in-shell apps).
- Interactive editing: click a pane (or F5 to cycle terminal -> chat ->
  roster) to select it; arrows then resize it live, Esc/Enter finishes.
  The selected pane gets a bold accent border and an ✎ title marker.
  ui::pane_at hit-tests against the same body_areas rects draw() paints.
- /layout slimmed to presets only (save/load/list/rm/reset); the old
  numeric pty/chat/roster and full/chatfull/normal verbs are replaced by
  the interactive flow.
- Help menu updated: LAYOUT cluster + KEYS line document F4, click/F5,
  arrows and Esc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/src/main.rs

@@ -7,6 +7,7 @@
 mod app;
 mod crypto;
 mod ft;
+mod layout;
 mod net;
 mod sbx;
 mod theme;