Show background effects as a grid in the camera menu

Adds the Background effects section from the design: no effect, blur, the
shipped images and a tile for adding your own, under the camera list.

The tiles are menuitemradio like the device rows above rather than a new
control, so the menu keeps one keyboard model throughout and the selection is
announced rather than only drawn. An image tile is the picture, filling the
tile, and its name goes to assistive technology alone; the add tile is a plus,
named the same way. The tick stands where the glyph would, carrying its own
ground on a picture so it reads against whatever is behind it.

Repurposing a menu item as a tile means overriding its row layout on three
axes, each found by measuring rather than by eye: it lays itself out as a grid
of chevron, icon and label, so flex-direction does nothing; it carries the gap
between them as a right margin on each, which pushes both off the tile's
centre when they are stacked; and it insets its own text sixteen pixels, so
the grid needs the same inset to line its tiles up with the heading above them.

Sized against the design file rather than judged: glyph and tick a quarter of
the tile wide, an ink gap of about a seventh of its height to the label, and
the tile's proportion carried by its padding. Rows are equal height, set by
the tallest tile — a fixed ratio clipped the label as soon as the menu was
narrow enough that the ratio could not hold it — and the columns carry a
minimum width so the menu grows wide enough for three tiles rather than
shrinking them to whatever the device names need.

The blur hatch is a stand-in: the design system has no blur icon, and the
design's own is an icon whose strokes shorten towards the corners. A uniform
hatch fills its box, so this one is drawn smaller and finer to carry the same
weight rather than the same size.

Where background processing is unavailable, no effect stays choosable: it
needs none and it is what the user has. The rest are disabled, and the images
lose their colour as well as their contrast, since a washed-out photograph
still reads as one you could pick.

The existing blur toggle stays for callers that pass no effects, so nothing
that renders this component has to change yet.

Exploration for FEATURES_SPEC/2026-09_Background_Effects.md. Not for merge: the
image tiles are stand-in gradients drawn in the story, and nothing wires the
grid to the pipeline yet.

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

locales/en/app.json

@@ -3,6 +3,7 @@
     "user_menu": "User menu"
   },
   "action": {
+    "add_background_image": "Add image",
     "blur_background": "Blur background",
     "close": "Close",
     "copy_link": "Copy link",
@@ -212,6 +213,7 @@
     "auto_gain_control_label": "Automatic gain control",
     "background_blur_header": "Background",
     "background_blur_label": "Blur the background of the video",
+    "background_effects_header": "Background effects",
     "bitrate_label": "Bitrate",
     "blur_not_supported_by_browser": "(Background blur is not supported by this device.)",
     "camera_header": "Camera quality",