test(unit): add a unit-test layer (backend cargo + frontend vitest)

The suite was almost entirely e2e; pure logic had no direct coverage. Add fast,
DB-free unit tests on both sides.

Backend (cargo test — inline #[cfg(test)] modules):
- rate_limiter: allow-up-to-max-then-block, per-key independence, sliding-window
  expiry, retry-after bounds, clear(), and client_ip X-Forwarded-For parsing
  (first entry / whitespace-trim / fallback).
- sse_tickets: single-use consume (replay → None), unknown ticket, uniqueness +
  hex shape, prune keeps fresh tickets, and an expired ticket (injected past-TTL
  entry) consumes to None.
- hashtag: fill the boundary gaps the 3 existing tests missed — stop-at-non-word,
  the 40-char cap (drops, doesn't truncate), no-dedup contract, and the German
  umlaut truncation limitation (pinned so a future Unicode fix is deliberate).

Frontend (Vitest — new, standalone config that stubs $app/environment so
server-safe module paths import cleanly under node):
- avatar: avatarPalette (neutral for empty, deterministic, real palette entry) and
  initials (?, single word, two words, whitespace collapse).
- data-mode-store: pickMediaUrl across original/saver modes and the
  preview→thumbnail→original fallback chain.
- `npm run test:unit` script added.

Backend: 20 passing. Frontend: 11 passing. svelte-check: 0 errors.

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

backend/src/models/hashtag.rs

@@ -109,4 +109,38 @@ mod tests {
     fn empty_or_bare_hash_skipped() {
         assert_eq!(extract_hashtags("# #"), Vec::<String>::new());
     }
+
+    #[test]
+    fn tag_stops_at_first_non_word_char() {
+        // A tag runs until the first char that isn't ascii-alphanumeric or '_'.
+        assert_eq!(extract_hashtags("#foo#bar"), vec!["foo"]);
+        assert_eq!(extract_hashtags("#foo-bar"), vec!["foo"]);
+        assert_eq!(extract_hashtags("##tag"), Vec::<String>::new()); // '#' after the strip is non-word
+    }
+
+    #[test]
+    fn tag_length_is_capped_at_40_chars() {
+        let ok = "a".repeat(40);
+        assert_eq!(extract_hashtags(&format!("#{ok}")), vec![ok.clone()]);
+        // 41+ chars → dropped entirely (not truncated).
+        let too_long = "a".repeat(41);
+        assert_eq!(extract_hashtags(&format!("#{too_long}")), Vec::<String>::new());
+    }
+
+    #[test]
+    fn duplicate_tags_are_returned_verbatim_not_deduplicated() {
+        // Dedup is the DB's job (Hashtag::upsert ON CONFLICT); extraction returns each
+        // occurrence so callers can count/link them independently. Case folds to lower.
+        assert_eq!(extract_hashtags("#fun #Fun #fun!"), vec!["fun", "fun", "fun"]);
+    }
+
+    #[test]
+    fn non_ascii_word_chars_truncate_the_tag() {
+        // KNOWN LIMITATION for a German app: `is_ascii_alphanumeric` excludes umlauts
+        // and ß, so a tag truncates at the first non-ASCII letter. Pinned here so a
+        // future Unicode-aware change is a deliberate, test-visible decision.
+        assert_eq!(extract_hashtags("#Grüße"), vec!["gr"]);
+        assert_eq!(extract_hashtags("#Straße"), vec!["stra"]);
+        assert_eq!(extract_hashtags("#café"), vec!["caf"]);
+    }
 }