fix(review): CSP nonce for FOUC script + feed_delta truncation signal

Two items surfaced by the branch re-review:

CSP regression (blocking): script-src 'self' blocked the template-authored
anti-FOUC theme script in app.html — SvelteKit's mode:'auto' only hashes
scripts it injects. Added nonce="%sveltekit.nonce%" so it's substituted per
request and included in the CSP (survives future edits, unlike a pinned hash).
Verified: emitted CSP nonce matches the script tag; no violation, no flash.

feed_delta silent truncation: the LIMIT 200 (added earlier to kill the
stale-`since` DoS) returned only the newest slice with no signal, so a client
that missed 200+ uploads during a reconnect could not tell it should full-
refresh — the older missed uploads were dropped and unrecoverable (the next
delta advances `since` past them). DeltaResponse now carries `truncated`; the
feed's feed-delta handler calls loadFeed(true) to resync from page 1 instead
of merging a partial slice.

Verified: cargo check clean, svelte-check 0 errors, production build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app.html

@@ -21,7 +21,10 @@
 			theme=dark don't flash a white screen. Mirrors the logic in
 			`src/lib/theme-store.ts`; kept in sync by hand (it's 6 lines).
 		-->
-		<script>
+		<!-- nonce is required: our CSP sets script-src 'self', and SvelteKit's
+		     mode:'auto' only hashes scripts *it* injects, not this template-authored
+		     one. %sveltekit.nonce% is substituted per request and added to the CSP. -->
+		<script nonce="%sveltekit.nonce%">
 			(function () {
 				try {
 					var pref = localStorage.getItem('eventsnap_theme') || 'system';