fix: guard tiling preview without grabbed view

[mateoalfaro]

Summary

labwc aborts with an assertion failure when a tiling drop preview is requested
without an active interactive move, taking down the whole session:

labwc: ../src/view.c:1399: view_is_floating: Assertion `view' failed.
(EE) failed to read Wayland events: Broken pipe
.singularity-labwc-session-wrapped: line 35: 23973 Aborted (core dumped) labwc -S "$BIN/singularity-desktop-session"
[...] labwc session exited rc=134 after 4s; retrying with software rendering

This repeats on every session start (~4s in, SIGABRT), and even the session
wrapper's software-rendering retry crashes with the same assertion, so the
desktop never comes up.

Root cause

view_is_floating() asserts on a non-NULL view, but two tiling-preview call
sites pass server.grabbed_view, which is only set during an interactive
move. Overlay updates can also be requested by the singularity-tiling protocol
while no move is active, so the NULL grabbed view reaches the assertion.

Changes

• src/interactive.c (edge_from_cursor): return early when there is no
  server.grabbed_view — with no grabbed view there is no edge-snap candidate.
• src/regions.c (regions_should_snap): add the same !server.grabbed_view
  guard alongside the existing snap conditions.
• src/protocols/singularity-tiling.c (handle_set_drop_preview): if the
  drop preview is not active, finish the overlay and return instead of calling
  overlay_update() on stale state.

Testing

• Rebuilt and ran the session on the affected machine: the assertion no longer
  triggers and the session stays up past the previous ~4s crash point.
• Verified view_is_floating() call sites: remaining callers either hold a
  valid view or already NULL-check.

Logs

Crash from ~/.local/state/singularity/labwc.log:

00:00:01.997 [ERROR] [xwayland/xwm.c:1984] xcb error: op ChangeWindowAttributes ...
labwc: ../src/view.c:1399: view_is_floating: Assertion `view' failed.
(EE) failed to read Wayland events: Broken pipe

Fix tested on NixOS 26.11