Add UI themes (Dark, Light, Midnight, Sunset)

Four preset color themes, switchable from Profile settings with instant
preview and server-side persistence (User.theme, applied via a data-theme
attribute the CSS custom-property overrides in themes.css key off). Dark
stays the existing DarkSingularity palette and default. Light is a genuine
new light-mode design; Midnight is a higher-contrast OLED-friendly dark
variant; Sunset swaps in a warm amber/coral accent family.

Custom theme building (pick-your-own-colors) is out of scope for this
pass -- presets only.

Also: fixed the PATCH /api/auth/me handler to only apply fields actually
present in the request body. It previously always overwrote display_name
unconditionally, which happened to be harmless when it was the only
field on ProfileUpdate but would have silently cleared it on any
theme-only update. And switched two hardcoded hex colors
(.btn-primary:hover, .role-badge-admin) to token-derived color-mix()
values so they adapt across themes instead of staying fixed to the
original cyan/violet palette.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/test_profile.py

@@ -53,6 +53,44 @@ async def test_display_name_too_long_rejected(client, db_session):
     assert resp.status_code == 422
 
 
+async def test_update_theme_persists(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+
+    resp = await client.patch("/api/auth/me", json={"theme": "midnight"})
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["theme"] == "midnight"
+
+    me = await client.get("/api/auth/me")
+    assert me.json()["theme"] == "midnight"
+
+
+async def test_invalid_theme_rejected(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+
+    resp = await client.patch("/api/auth/me", json={"theme": "not-a-real-theme"})
+    assert resp.status_code == 422
+
+
+async def test_updating_theme_does_not_clobber_display_name(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    await client.patch("/api/auth/me", json={"display_name": "Alice A."})
+
+    resp = await client.patch("/api/auth/me", json={"theme": "light"})
+    assert resp.status_code == 200
+    assert resp.json()["display_name"] == "Alice A."
+    assert resp.json()["theme"] == "light"
+
+
+async def test_updating_display_name_does_not_clobber_theme(client, db_session):
+    await register_and_login(client, db_session, username=_unique("alice"))
+    await client.patch("/api/auth/me", json={"theme": "sunset"})
+
+    resp = await client.patch("/api/auth/me", json={"display_name": "Alice A."})
+    assert resp.status_code == 200
+    assert resp.json()["theme"] == "sunset"
+    assert resp.json()["display_name"] == "Alice A."
+
+
 async def test_avatar_upload_succeeds_and_persists(client, db_session):
     await register_and_login(client, db_session, username=_unique("alice"))