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>

tests/test_sprint26.py

@@ -0,0 +1,109 @@
+"""
+Sprint 26 Tests: pluggable UI themes — settings persistence, theme default,
+custom theme names accepted.
+"""
+import json, urllib.error, urllib.request
+
+BASE = "http://127.0.0.1:8788"
+
+
+def get(path):
+    with urllib.request.urlopen(BASE + path, timeout=10) as r:
+        return json.loads(r.read()), r.status
+
+
+def post(path, body=None):
+    data = json.dumps(body or {}).encode()
+    req = urllib.request.Request(BASE + path, data=data,
+                                headers={"Content-Type": "application/json"})
+    try:
+        with urllib.request.urlopen(req, timeout=10) as r:
+            return json.loads(r.read()), r.status
+    except urllib.error.HTTPError as e:
+        return json.loads(e.read()), e.code
+
+
+# ── Theme settings ───────────────────────────────────────────────────────
+
+def test_settings_default_theme():
+    """Default theme should be 'dark'."""
+    d, status = get("/api/settings")
+    assert status == 200
+    assert d.get("theme") == "dark"
+
+
+def test_settings_set_theme_light():
+    """Setting theme to 'light' should persist and round-trip."""
+    try:
+        d, status = post("/api/settings", {"theme": "light"})
+        assert status == 200
+        d2, _ = get("/api/settings")
+        assert d2.get("theme") == "light"
+    finally:
+        # Reset to dark
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_solarized():
+    """Setting theme to 'solarized' should persist."""
+    try:
+        post("/api/settings", {"theme": "solarized"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "solarized"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_monokai():
+    """Setting theme to 'monokai' should persist."""
+    try:
+        post("/api/settings", {"theme": "monokai"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "monokai"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_nord():
+    """Setting theme to 'nord' should persist."""
+    try:
+        post("/api/settings", {"theme": "nord"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "nord"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_custom_theme_accepted():
+    """Custom theme names should be accepted (no enum gate)."""
+    try:
+        d, status = post("/api/settings", {"theme": "my-custom-theme"})
+        assert status == 200
+        d2, _ = get("/api/settings")
+        assert d2.get("theme") == "my-custom-theme"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_theme_does_not_break_other_settings():
+    """Setting theme should not disturb other settings."""
+    d_before, _ = get("/api/settings")
+    send_key_before = d_before.get("send_key")
+    try:
+        post("/api/settings", {"theme": "nord"})
+        d_after, _ = get("/api/settings")
+        assert d_after.get("send_key") == send_key_before
+        assert d_after.get("theme") == "nord"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_theme_survives_round_trip():
+    """Theme set via POST should appear in subsequent GET."""
+    try:
+        post("/api/settings", {"theme": "monokai"})
+        d, status = get("/api/settings")
+        assert status == 200
+        assert d["theme"] == "monokai"
+    finally:
+        post("/api/settings", {"theme": "dark"})