perf(ui): deprioritise the dictionary download; drop the lobby prefetch

On a slow link (EDGE) the background dictionary download starved the game's own
traffic — the board and the move preview stalled behind a 450KB fetch. Fetch the
dawg at low priority (Priority Hints; ignored on iOS WebKit, harmless) so the
browser schedules it behind the game's requests, and load it only on game open —
the lobby no longer prefetches, which was its main competitor for the channel.
Evict a cached blob the reader rejects (a stale or partial entry) so it self-heals
instead of lingering and re-fetches. Raise the app-entry bundle budget to 110KB
(the feature's small in-entry game/debug wiring; the heavy dict code stays lazy).

docs/UI_DESIGN.md

@@ -429,9 +429,10 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
 
 ## Dictionary warm-up overlay (`components/DictWarmup.svelte`)
 
-Opening a game whose local move-preview dictionary is not yet in memory shows a
-brief, non-dismissable warm-up overlay while it loads (the lobby pre-warms the
-player's ongoing games, so it rarely appears). A darker-than-onboarding scrim covers
+Opening a game whose local move-preview dictionary is not yet cached shows a brief,
+non-dismissable warm-up overlay while it downloads (a returning player's cached
+dictionary loads from IndexedDB in a few ms, so the flash-guard suppresses it then).
+A darker-than-onboarding scrim covers
 the board; centred on it, a large emoji cycles through a fixed playful sequence — one
 per 500 ms tick (300 ms still, then a 200 ms clockwise spin with the current glyph
 fading out and the next fading in), looping — under a constant "Loading…" caption.