feat(settings): Schriftfarbe, Linienfarbe und Hintergrundfarbe per Color-Picker

Die bisherigen Stufen-Wähler ("Dunkel/Mittel/Hell/Maximum" und
"Kaum/Subtil/Normal/Stark") für Schrift- bzw. Linienkontrast sind durch
echte Hex-Color-Picker ersetzt. Zusätzlich kann jetzt auch die
Hintergrundfarbe der Seite frei gewählt werden.

Wenn ein Override gesetzt ist:
- text_color → setzt --text-1 direkt, --text-2/--text-3 werden
  daraus per shadeHex(-0.25 / -0.55) abgeleitet, damit der Hue passt
- line_color → setzt --border, --border-light wird leicht abgedunkelt
- bg_color → setzt --bg-app, daraus werden Topbar/Sidebar/Surface/
  Hover/Active per shadeHex(+0.10…+0.40) konsistent hochskaliert

Per "Reset"-Knopf wird der Override geleert und die alte Stufen-Logik
(falls noch vorhanden) bzw. der Default-Theme greift wieder.

Backend:
- 3 neue nullable VARCHAR(7)-Spalten in user_settings (text_color,
  line_color, bg_color) inkl. Migrationen in main.py
- settings_router nutzt model_dump(exclude_unset=True) und respektiert
  explizite null-Werte nur für diese 3 Override-Felder, damit Reset
  funktioniert

Auch enthalten: Auflösen der Merge-Konflikte in sw.js, index.html,
version.js (HEAD-Stand v17 behalten) und Bump auf v18.

frontend/sw.js

@@ -1,4 +1,3 @@
-<<<<<<< HEAD
 // Calendarr Service Worker — minimal-cache strategy
 //
 // Strategy: network-first for everything. The cache is only used as a
@@ -8,52 +7,15 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v17';
+const CACHE_VERSION  = 'calendarr-v18';
 const OFFLINE_SHELL  = ['/', '/index.html'];
-=======
-// Calendarr Service Worker
-// Cache-first for static assets, network-first for /api/* (graceful offline)
-
-const CACHE_VERSION = 'calendarr-v11';
-const STATIC_ASSETS = [
-  '/',
-  '/index.html',
-  '/manifest.json',
-  '/static/css/app.css',
-  '/static/favicon.svg',
-  '/static/js/app.js',
-  '/static/js/api.js',
-  '/static/js/calendar.js',
-  '/static/js/color-picker.js',
-  '/static/js/date-picker.js',
-  '/static/js/i18n.js',
-  '/static/js/utils.js',
-  '/static/js/version.js',
-  '/static/js/views/agenda.js',
-  '/static/js/views/month.js',
-  '/static/js/views/quarter.js',
-  '/static/js/views/week.js',
-  '/icons/icon-192.png',
-  '/icons/icon-512.png',
-  '/icons/icon.svg',
-];
->>>>>>> e744b1829e99db6b80922f75542ced329138e474
 
 self.addEventListener('install', event => {
   event.waitUntil(
     caches.open(CACHE_VERSION).then(cache =>
-<<<<<<< HEAD
       Promise.all(OFFLINE_SHELL.map(url =>
         cache.add(url).catch(err => console.warn('[SW] skip', url, err))
       ))
-=======
-      // Use addAll with a fallback so a single missing file doesn't abort install
-      Promise.all(
-        STATIC_ASSETS.map(url =>
-          cache.add(url).catch(err => console.warn('[SW] skip', url, err))
-        )
-      )
->>>>>>> e744b1829e99db6b80922f75542ced329138e474
     ).then(() => self.skipWaiting())
   );
 });
@@ -72,12 +34,8 @@ self.addEventListener('fetch', event => {
 
   const url = new URL(req.url);
 
-<<<<<<< HEAD
   // API routes: always go to the network, no offline fallback (we'd just
   // be returning stale account/event data otherwise).
-=======
-  // Network-first for API routes — fail silently if offline
->>>>>>> e744b1829e99db6b80922f75542ced329138e474
   if (url.pathname.startsWith('/api/')) {
     event.respondWith(
       fetch(req).catch(() =>
@@ -90,7 +48,6 @@ self.addEventListener('fetch', event => {
     return;
   }
 
-<<<<<<< HEAD
   // Everything else: network-first. The browser's HTTP cache (driven by
   // the server's Cache-Control headers) already throttles re-fetches —
   // the SW just makes sure offline still works for the entry HTML.
@@ -114,47 +71,6 @@ self.addEventListener('fetch', event => {
         return caches.match(req).then(c => c || caches.match('/index.html'));
       }
       return new Response('', { status: 503 });
-=======
-  // Network-first for navigation (HTML) and the version-defining files —
-  // ensures users always get the freshest entry point so new releases
-  // take effect on the next reload without a manual SW unregister.
-  const isHtml = req.mode === 'navigate'
-    || url.pathname === '/'
-    || url.pathname === '/index.html';
-  const isVersionFile = url.pathname === '/static/js/version.js';
-
-  if (isHtml || isVersionFile) {
-    event.respondWith(
-      fetch(req).then(resp => {
-        if (resp && resp.status === 200) {
-          const clone = resp.clone();
-          caches.open(CACHE_VERSION).then(c => c.put(req, clone)).catch(() => {});
-        }
-        return resp;
-      }).catch(() =>
-        caches.match(req).then(c => c || caches.match('/index.html'))
-      )
-    );
-    return;
-  }
-
-  // Cache-first for everything else (static)
-  event.respondWith(
-    caches.match(req).then(cached => {
-      if (cached) return cached;
-      return fetch(req).then(resp => {
-        // Only cache successful, basic-origin responses
-        if (resp && resp.status === 200 && resp.type === 'basic') {
-          const clone = resp.clone();
-          caches.open(CACHE_VERSION).then(c => c.put(req, clone)).catch(() => {});
-        }
-        return resp;
-      }).catch(() => {
-        // Offline fallback for navigation requests
-        if (req.mode === 'navigate') return caches.match('/index.html');
-        return new Response('', { status: 503 });
-      });
->>>>>>> e744b1829e99db6b80922f75542ced329138e474
     })
   );
 });