feat: sessions management, web manifest, square icon-only buttons, remove lang select

- Backend: migration 002 adds user_agent/ip/last_used_at to refresh_tokens
- Backend: GET /api/auth/sessions + DELETE /api/auth/sessions/{id} endpoints
- Frontend: profile page — sessions section (browser, IP, datetime, revoke)
- Frontend: web manifest + SVG icon for PWA support
- Frontend: remove language selector from all navbars (moved to profile page)
- Frontend: neu-btn--icon-sm class for square icon-only buttons (theme/logout/edit)
- Frontend: manifest link added to all 9 HTML pages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/icons/icon.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="bg" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#1a1a2e"/>
+      <stop offset="100%" style="stop-color:#16213e"/>
+    </linearGradient>
+    <linearGradient id="accent" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#6c8ef4"/>
+      <stop offset="100%" style="stop-color:#a78bfa"/>
+    </linearGradient>
+  </defs>
+  <!-- Background rounded square -->
+  <rect width="512" height="512" rx="96" fill="url(#bg)"/>
+  <!-- Server rack lines -->
+  <rect x="96" y="140" width="320" height="64" rx="12" fill="none" stroke="url(#accent)" stroke-width="10" opacity="0.4"/>
+  <rect x="96" y="224" width="320" height="64" rx="12" fill="none" stroke="url(#accent)" stroke-width="10" opacity="0.6"/>
+  <rect x="96" y="308" width="320" height="64" rx="12" fill="none" stroke="url(#accent)" stroke-width="10" opacity="0.4"/>
+  <!-- Status dots -->
+  <circle cx="142" cy="172" r="8" fill="#22c55e"/>
+  <circle cx="142" cy="256" r="8" fill="url(#accent)"/>
+  <circle cx="142" cy="340" r="8" fill="#22c55e"/>
+  <!-- P letter -->
+  <text x="256" y="300" text-anchor="middle" font-family="system-ui, sans-serif" font-weight="800" font-size="200" fill="url(#accent)" opacity="0.12">P</text>
+</svg>