GroupByDate : group by day

[trecoool]

Hello!

In Immich, photos are grouped per day which is how i'd like to share my public albums through Immich Public Proxy.
I've seen the GroupByDate setting which i have enabled but it groups per month.

I'd like to do a pull request to add the ability to set a per day grouping, thanks to my dear friend Claude, following that plan :

Add day-level date grouping (groupByDate: "day")
Config shape. Widen ipp.gallery.groupByDate from a boolean to false | "month" | "day":

• false (default) — no grouping (unchanged)
• true / "month" — month headers (unchanged; true normalized to "month")
• "day" — one header per day
• Grouping rule — follows Immich. Bucket and sort by each asset's local date (localDateTime, already returned by the timeline API; fall back to fileCreatedAt if absent), so a photo appears under the day it was taken regardless of viewer timezone — same as the Immich timeline. Month mode keeps its current (UTC-prefix) behavior.

Implementation (pure client-side layout + passing one field/flag through — no new Immich calls, no state):

client/layout.ts — generalize groupItemsByMonth() → groupItemsByDate(unit): slice YYYY-MM vs YYYY-MM-DD; label adds the day via Intl.DateTimeFormat.
gallery/builder.ts — normalize the config value to "month" | "day" | null, sort accordingly, forward the mode in the init JSON.
view/gallery.tsx, shared/types.ts, client/state.ts, client/init.ts — carry the "month" | "day" mode to the client (replaces the current bool).
types.ts (TimelineBucketAssets) + immich.ts (timelineBucketToAssets) + GalleryItem — add localDateTime so grouping uses local date.
config.json default stays false; update the groupByDate row in README.md + docs/configuration.md.
Backward compatibility. Existing groupByDate: true configs keep working (normalized to "month"). No key rename, no migration shim.

Tests. Add unit coverage for groupItemsByDate (month vs day bucketing, undated handling) — the first tests for layout.ts.

Let me know what you think!
Thank you

[alangrainger]

Done in c2fcb81 This will go live in v3.0.3