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/Tile-Grid.md

@@ -65,8 +65,10 @@ whose agent exited inside its pane says so instead; close that session from `⋯
 ## Moving tiles
 
 Drag a tile by its header (anywhere but its buttons) onto another tile and the two trade
-places. Drop it on an empty slot and it moves there, to the end of the grid. Press `Escape`
-or let go anywhere else and nothing moves. The dropped tile takes the focus.
+places; the dropped tile takes the focus. Empty slots are for adding sessions (drop a tab
+there), so a tile cannot be dropped on one. Press `Escape` or let go anywhere else and nothing
+changes, not even which tile has the focus: a header focuses its tile when you click it, not
+when you press it.
 
 With the keyboard, `Ctrl+Shift+Arrows` moves the focused tile left, right, up or down: it
 trades places with the tile next to it (the one `Alt+Shift+Arrows` would focus) and keeps the
@@ -90,10 +92,10 @@ saved with the grid.
 
 All of them can be rebound in App Settings → Shortcuts, where **Remove Focused Tile** can
 also get a key. Outside the grid, `Alt+Shift+Arrows`, `Ctrl+Shift+Arrows` and
-`Alt+Shift+Enter` go to the terminal as usual. While it is open, `Ctrl+Shift+Arrows` in a text
-field (renaming a tile, the file editor) still selects by word; inside a tile it moves the tile,
-so a terminal editor there (nano, micro, emacs) does not get it. With the Tiles setting off,
-`Ctrl+Shift+G` does nothing.
+`Alt+Shift+Enter` go to the terminal as usual. While it is open, `Alt+Shift+Arrows` and
+`Ctrl+Shift+Arrows` in a text field (renaming a tile, the file editor) still select text there;
+inside a tile they focus and move tiles, so a terminal editor there (nano, micro, emacs) does
+not get them. With the Tiles setting off, `Ctrl+Shift+G` does nothing.
 
 ## Leaving the grid