Fix the review's findings: drag on pointer events, read the window back

Ten findings from the review of the previous commit, all applied.

**The tab drag is now pointer events, not HTML5 drag-and-drop.** Two
independent reasons, either one fatal. Tauri's `dragDropEnabled` blocks
HTML5 drag inside the webview on Windows, and it cannot just be turned
off — `TerminalView` needs Tauri's native drag-drop event, which is the
only one that carries dropped *file paths*. And an HTML5 drag carries a
`DataTransfer`: released over any text field in the app, the default
handler types `term:<uuid>` into it, and in Config that is then saved
with the project. Pointer events have neither problem, and the drag is
measured from the tabs on screen rather than from the event target, so
the marker and the drop agree even over the marker itself. Escape
abandons a drag; a press under 4px stays a click; the click that ends a
drag does not select.

**`Ctrl+Shift+←/→` no longer swallows word-wise selection.** It is bound
on `document` in the capture phase, so in any input — the rename field,
Config, Settings — it was taking the OS's extend-selection chord *and*
silently reordering the strip. Guarded by `inTextField()`, which
excludes xterm's helper textarea: that is an input-method shim, and the
terminal is where the shortcut matters most.

**The pop-out's state is read from the window, never remembered.** The
pane is unmounted whenever another Project Home sub-tab is selected, so
"Keep on top" came back Off over a window still floating on top.
`get_browser_view_popout_state` returns both facts from the window
itself, and the change event carries them. `poppedOut` is tri-state:
until the answer arrives the iframe is not mounted, because guessing
"not popped out" is what flashes a second viewer onto the browser.

Also: `popout::close` and the off-status emit in the supervisor are
behind the same epoch guard as the deregistration above them, so a
supervisor whose teardown outlives a restart can no longer destroy the
*new* session's window; `close()` returns its `destroy()` error instead
of logging it and reporting success, since the pane restores its iframe
on success; the drop marker is `pointer-events-none` and is placed
before the first *visible* tab at or past the slot, so it neither
refuses a drop nor vanishes when a `tabOrder` entry renders nothing; and
the "Keep on top" Toggle's accessible name now matches its visible text.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

HOW-TO-USE.md

@@ -192,10 +192,11 @@ Anthropic-backend project uses that token without its own login. See
   There is no separate terminal tab bar and no "+" button — tabs appear when you open a project or
   a terminal.
 
-  **Drag a tab to reorder it.** A line shows where it will land, and dropping it does not change
-  which tab you are looking at — so you can rearrange the strip without pulling focus away from a
-  terminal that is mid-run. `Ctrl+Shift+←` and `Ctrl+Shift+→` move the *active* tab the same way
-  without the mouse. The order is per-session: it is not saved when you quit.
+  **Drag a tab to reorder it.** A line shows where it will land; **Escape** abandons the drag.
+  Dropping does not change which tab you are looking at — so you can rearrange the strip without
+  pulling focus away from a terminal that is mid-run. `Ctrl+Shift+←` and `Ctrl+Shift+→` move the
+  *active* tab the same way without the mouse (they leave text fields alone, where that chord
+  still selects by word). The order is per-session: it is not saved when you quit.
 - **Status indicators (top right)** — Docker connection and container image availability. Each pairs
   a coloured dot with a word, so status is never conveyed by colour alone. The **?** button opens
   the built-in help.