feat: Sprint 26 — pluggable UI themes (dark, light, solarized, monokai, nord)

Five built-in themes with instant switching, persistent preference,
and zero-flicker loading. Custom themes are pure CSS additions.

Theme system:
- CSS variable overrides via :root[data-theme="name"] blocks
- Flicker prevention: inline <script> reads localStorage before
  stylesheet parses, preventing dark-flash on light-mode users
- Server-side persistence via settings.json (theme field)
- Boot.js syncs server preference to DOM + localStorage

Built-in themes:
- Dark (default): deep navy/indigo, muted blue accents
- Light: clean white/gray, high contrast, scrollbar overrides
- Solarized Dark: teal background, warm accents
- Monokai: warm dark, green/pink accents
- Nord: arctic blue-gray, calm and minimal

UI integration:
- Settings panel: theme dropdown with instant live preview
- /theme slash command: /theme dark|light|solarized|monokai|nord
- No enum constraint on theme setting — custom themes just work

Documentation:
- THEMES.md: how to switch themes, create custom themes, contribute

8 new tests. All 408 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/style.css

@@ -4,6 +4,29 @@
     --text:#e8e8f0;--muted:#8888aa;--accent:#e94560;--blue:#7cb9ff;--gold:#c9a84c;--code-bg:#0d1117;
     font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",system-ui,sans-serif;font-size:14px;line-height:1.6;
   }
+  /* ── Light theme ── */
+  :root[data-theme="light"]{
+    --bg:#f5f5f7;--sidebar:#e8e8ed;--border:rgba(0,0,0,0.10);--border2:rgba(0,0,0,0.16);
+    --text:#1c1c1e;--muted:#6e6e80;--accent:#c0392b;--blue:#0a6dc2;--gold:#a07a20;--code-bg:#f0f0f5;
+  }
+  :root[data-theme="light"] ::-webkit-scrollbar-thumb{background:rgba(0,0,0,.15);}
+  :root[data-theme="light"] ::-webkit-scrollbar-thumb:hover{background:rgba(0,0,0,.3);}
+  :root[data-theme="light"] ::selection{background:rgba(10,109,194,.2);}
+  /* ── Solarized Dark theme ── */
+  :root[data-theme="solarized"]{
+    --bg:#002b36;--sidebar:#073642;--border:rgba(255,255,255,0.08);--border2:rgba(255,255,255,0.13);
+    --text:#839496;--muted:#657b83;--accent:#dc322f;--blue:#268bd2;--gold:#b58900;--code-bg:#073642;
+  }
+  /* ── Monokai theme ── */
+  :root[data-theme="monokai"]{
+    --bg:#272822;--sidebar:#1e1f1c;--border:rgba(255,255,255,0.07);--border2:rgba(255,255,255,0.12);
+    --text:#f8f8f2;--muted:#75715e;--accent:#f92672;--blue:#66d9e8;--gold:#e6db74;--code-bg:#1e1f1c;
+  }
+  /* ── Nord theme ── */
+  :root[data-theme="nord"]{
+    --bg:#2e3440;--sidebar:#272c36;--border:rgba(255,255,255,0.07);--border2:rgba(255,255,255,0.12);
+    --text:#eceff4;--muted:#9099aa;--accent:#bf616a;--blue:#81a1c1;--gold:#ebcb8b;--code-bg:#272c36;
+  }
   body{background:var(--bg);color:var(--text);height:100vh;height:100dvh;overflow:hidden;display:flex;}
   .layout{display:flex;width:100%;height:100vh;height:100dvh;}
   .sidebar{width:300px;background:var(--sidebar);border-right:1px solid var(--border);display:flex;flex-direction:column;overflow:visible;flex-shrink:0;}