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>

frontend/src/test/mocks/app-environment.ts

@@ -0,0 +1,7 @@
+// Test stub for SvelteKit's `$app/environment` virtual module (which only exists
+// during dev/build). `browser: false` makes modules like data-mode-store take their
+// server-safe path (no localStorage access) when imported under Vitest's node env.
+export const browser = false;
+export const dev = false;
+export const building = false;
+export const version = 'test';