Display mode: crop to fill, crop to faces, stretch to fill, shrink to fit

Replaces the smart_crop_faces boolean with a 4-way display_mode select
on each frame's Configuration tab (image_pipeline.DISPLAY_MODES):

- Crop to fill / Crop to faces: the previous False/True behavior,
  unchanged (center-crop trimming excess, optionally shifted to keep
  faces on screen).
- Stretch to fill (new): fills the panel exactly, aspect ratio not
  preserved -- a plain resize, no crop.
- Shrink to fit (new): the whole photo visible, letterboxed with white
  where it doesn't fill the panel.

Named-face overlay label positioning (face_labels.py, the manage menu's
"who's in this photo") now goes through a shared _placement_transform()
in image_pipeline.py instead of duplicating crop-box math, so label
placement stays correct (and in-bounds) under all four modes, not just
the two crop ones -- letterbox/stretch never crop a face out, so labels
just use straight scale+offset math there.

Schema migration v5 adds display_mode, backfills it from the old
boolean (True/False -> crop_faces/crop_fill), and drops the boolean.

Verified against the live-shaped test database: the migration
(existing frames correctly preserved as crop_faces), the config page's
new 4-option select, actual renders under letterbox (confirmed real
white letterbox padding in the packed panel-code bytes) and
stretch_fill, invalid-input fallback, and the standing legacy-device
curl suite.

server/app/routers/frame_pages.py

@@ -12,7 +12,12 @@ from sqlalchemy.orm import Session
 
 from ..auth import can_view_frame, current_user
 from ..db import get_db
-from ..image_pipeline import DEFAULT_PALETTE_RGB, PALETTE_LABELS, palette_to_hex
+from ..image_pipeline import (
+    DEFAULT_PALETTE_RGB,
+    DISPLAY_MODE_LABELS,
+    PALETTE_LABELS,
+    palette_to_hex,
+)
 from ..models import Frame
 from ..quiet_hours import ALL_TIMEZONES
 from .common import shell_context
@@ -46,6 +51,7 @@ def frame_config_page(frame_id: int, request: Request, db: Session = Depends(get
         palette_labels=PALETTE_LABELS,
         default_palette_rgb=DEFAULT_PALETTE_RGB,
         palette_to_hex=palette_to_hex,
+        display_mode_labels=DISPLAY_MODE_LABELS,
     )