Add remove-from-rotation, fix mobile scroll-vs-drag conflict

Remove from rotation: a new bounded exclude list
(FrameConfig.excluded_asset_ids) that photo_queue._top_up() never
selects from. POST /api/queue/remove scrubs an asset out of
queue/history too so it can't resurface via "Show next" or the back
button, and if it was the current photo, advances away from it
immediately -- without recording it in history, since going back to a
photo you just explicitly removed doesn't make sense. Doesn't touch
Immich or the album itself, just this frame's own selection. Wired into
the web UI as a small "x" button on both the current-photo thumbnail
and every upcoming card.

Mobile scroll fix: touching a card to scroll the page was being
captured as a drag attempt every time (touch-action: none on every
.photo-card, needed for the existing drag-reorder gesture to work at
all), making it too easy to accidentally reorder instead of scroll.
Reworked touch dragging to require a brief hold (350ms, roughly
stationary) before it arms -- touch-action stays "pan-y" (native
scroll allowed) the whole time up to that point, so a normal
touch-and-swipe scrolls the page like anywhere else, and only switches
to "none" once a hold is confirmed as deliberate. Mouse dragging is
unchanged (no hold delay -- no scroll-vs-drag ambiguity with a mouse).
Also made the "Show next" and new remove buttons always visible instead
of hover/focus-revealed, since that was invisible-but-still-tappable on
touch (no hover state) -- a real hazard for a destructive action.

server/app/config.py

@@ -37,6 +37,7 @@ class FrameConfig(BaseModel):
     queue_cursor: int = 0  # internal bookkeeping for sequential queue top-up; not user-facing
     queue_target_len: int = 20  # how many upcoming photos to keep queued/shown in the web UI
     history: list[str] = []  # bounded stack of previously-current asset ids, most recent last
+    excluded_asset_ids: list[str] = []  # permanently removed from this frame's rotation (not deleted from Immich)
 
 
 def load() -> FrameConfig: