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>

docs/wiki/Keyboard-Shortcuts.md

@@ -46,7 +46,7 @@ Anything you copy is cleaned on the way to the clipboard: each line loses the pa
 | `Ctrl+Shift+G`              | Open or close the tile grid (needs the Tiles setting on).     |
 | `Alt+Shift+Arrows`          | Focus the tile to the left, right, above or below.            |
 | `Ctrl+Shift+Arrows`         | Move the focused tile: it trades places with that neighbour.  |
-| Drag a tile's header        | Move the tile: onto another tile they swap, onto an empty slot it moves there. |
+| Drag a tile's header        | Move the tile: dropped on another tile, the two swap.         |
 | `Alt+Shift+Enter`           | Zoom the focused tile, or restore the grid.                   |
 | `Ctrl`+click / `Cmd`+click a tab | Add that session to the grid.                            |
 | Right-click the Tiles button | Choose which sessions to show as tiles.                  |