fix(tiles): no tile onto an empty slot, header focus on click, arrow chords skip text fields

The owner's answers on moving tiles:

- "dont move the tile": an empty slot no longer takes a tile. A slot is
  always the last cell, so a move there shifted every tile after it.
  Each drop target now says what it accepts (_acceptTabDrops'
  `accepts`): a tile takes any session but its own, an empty slot only a
  session not tiled yet. A refused drag is still held (dropEffect none,
  no highlight), and dropSessionOnSlot refuses a tiled session too, its
  tab included. A session not tiled yet still joins on a slot.
- A cancelled drag changes nothing, focus included (best practice): the
  header focuses its tile on click, never on press, so a drag that ends
  with Escape or outside leaves focus and the idle alert alone. The body
  keeps press-to-focus, so focus still moves before a press reaches
  xterm. The rename input stops its own clicks.
- A tiled tab dropped on the zoomed tile stays refused (confirmed).
- The Alt+Shift+Arrow focus chords skip a text field too (best
  practice), as the move chords already did: shifted arrows select
  there. Toggle and zoom are not text-editing keys and are unchanged.

Docs: the wiki, the spec's as-built bullet (with the owner's answers),
the invariants and CLAUDE.md say so.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -543,7 +543,9 @@ const DEFAULT_SHORTCUTS = [
   // chord must reach the terminal untouched. Every xterm key handler swallows a
   // chord that applies, so it never reaches a PTY. Defaults: Ctrl+Shift+G makes
   // xterm emit nothing (a shifted Ctrl letter) and overrides only the browser's
-  // find-previous; Alt+Shift+Arrows are bound by no CLI Codeman runs.
+  // find-previous; Alt+Shift+Arrows are bound by no CLI Codeman runs. The
+  // arrow chords (focus, move) never apply in a text field, where shifted
+  // arrows select (tileShortcutFor).
   {
     id: 'toggle-tile-grid',
     group: 'Tiles',