refactor(calendar): stack layout, wider grid, fixed cell height

- Calendar widget stays stacked (calendar above events list)
- Grid widens to max-w-2xl with fixed h-10 cells (no aspect-square height growth)
- Gap between cells: 0.5 → 1.5
- Button system extracted to shared btn-* utility classes in app.css
- Replaced Google Calendar link + Google Maps with OSM
- Removed google icon from iconify subset

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

src/app.css

@@ -279,12 +279,12 @@ h4 {
 }
 
 /* Content-Links: Himmel (Design System – Links) */
-main a:not(.no-underline) {
+main a:not(.no-underline):not([class*="btn"]) {
     color: var(--color-link);
     text-decoration: underline;
 }
 
-main a:not(.no-underline):hover {
+main a:not(.no-underline):not([class*="btn"]):hover {
     color: var(--color-himmel-600);
 }
 
@@ -317,9 +317,17 @@ main ol:not(.not-prose) li {
     list-style-type: decimal;
 }
 
+/* Buttons – gemeinsame Basis */
+.btn-primary,
+.btn-secondary,
+.btn-tertiary,
+.btn-outline {
+    @apply inline-flex items-center gap-1 no-underline whitespace-nowrap transition-colors duration-200 font-medium px-4 py-2 rounded-xs;
+}
+
 /* Buttons – Wald (Primary) */
 .btn-primary {
-    @apply inline-block no-underline transition-colors duration-200 font-medium px-4 py-2 rounded-xs shadow-sm bg-btn-bg text-btn-txt;
+    @apply shadow-sm bg-btn-bg text-btn-txt;
 }
 
 .btn-primary:hover {
@@ -328,13 +336,36 @@ main ol:not(.not-prose) li {
 
 /* Buttons – Stein outline (Secondary) */
 .btn-secondary {
-    @apply inline-block no-underline transition-colors duration-200 font-medium px-4 py-2 rounded-xs border border-stein-300 bg-white text-stein-700 shadow-sm;
+    @apply border border-stein-300 bg-white text-stein-700 shadow-sm;
 }
 
 .btn-secondary:hover {
     @apply bg-stein-100 border-stein-400;
 }
 
+/* Buttons – Himmel (Tertiary) */
+.btn-tertiary {
+    @apply shadow-sm bg-himmel-700 text-white;
+}
+
+.btn-tertiary:hover {
+    @apply bg-himmel-800;
+}
+
+/* Buttons – Stein-Border klein (Outline) */
+.btn-outline {
+    @apply border border-stein-300 text-stein-700;
+}
+
+.btn-outline:hover {
+    @apply bg-stein-100;
+}
+
+/* Größen-Modifier — nach den btn-* definiert damit override greift */
+.btn-sm {
+    @apply px-2 py-1 text-xs gap-1;
+}
+
 /* Dekorativer Hintergrund mit zwei verschwommenen Verlaufs-Blobs (Wald → Erde).
    Nutzung: Container mit relativer Positionierung; Blobs sitzen in ::before/::after.
    Inhalt sollte z-Index >= 0 haben (Standard); Blobs liegen unter Stacking-Kontext. */
@@ -571,30 +602,6 @@ footer a:hover,
     background: var(--color-container-breakout);
 }
 
-/* Kalender: Container Query – links Kalender, rechts Eventblock ab ~500px Containerbreite */
-.calendar-block {
-    container-type: inline-size;
-    container-name: calendar;
-}
-@container calendar (min-width: 600px) {
-    .calendar-block .calendar-grid-wrapper {
-        grid-template-columns: auto 1fr;
-    }
-    .calendar-block .calendar-column {
-        min-width: 300px;
-        max-width: 450px;
-    }
-    .calendar-block .calendar-events-wrapper {
-        position: relative;
-    }
-    .calendar-block .calendar-events-panel {
-        position: absolute;
-        inset: 0;
-        border-top: none;
-        border-left: 1px solid var(--color-stein-200);
-    }
-}
-
 /* ── Print rules ────────────────────────────────────────────────────────────
    Goal: print only the article. `print:hidden` on chrome (header/footer/
    banner/breadcrumbs/comments) handles structural removal. The rules below