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/lib/data-mode-store.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { pickMediaUrl } from './data-mode-store';
+
+const upload = {
+  id: 'abc-123',
+  preview_url: '/media/previews/p.jpg',
+  thumbnail_url: '/media/thumbnails/t.jpg',
+};
+
+describe('pickMediaUrl', () => {
+  it('original mode → the original API route, ignoring preview/thumbnail', () => {
+    expect(pickMediaUrl('original', upload)).toBe('/api/v1/upload/abc-123/original');
+  });
+
+  it('saver mode → preview_url when present', () => {
+    expect(pickMediaUrl('saver', upload)).toBe('/media/previews/p.jpg');
+  });
+
+  it('saver mode → thumbnail_url when preview is null', () => {
+    expect(pickMediaUrl('saver', { ...upload, preview_url: null })).toBe('/media/thumbnails/t.jpg');
+  });
+
+  it('saver mode → original route when both preview and thumbnail are null', () => {
+    expect(pickMediaUrl('saver', { id: 'x', preview_url: null, thumbnail_url: null })).toBe(
+      '/api/v1/upload/x/original'
+    );
+  });
+});