fix: keep active page on config save, add Cyrillic preview support, fix grid sizing

- daemon.go: save/restore active page on config reload instead of
  always activating default_page (fixes page switch on save)
- render.go: replace basicfont.Face7x13 with parseDisplayFace() for
  unicode/Cyrillic support in edit key preview
- web.go: add SSE endpoint (GET /api/events) for real-time page_changed
  events and POST /api/activate-page API
- app.js: persist active page via localStorage, listen for SSE
  page_changed events
- styles.css, index.html: fix grid sizing (width 100%, minmax(0, 1fr),
  aspect-ratio on key buttons, max-width 525px)

static/styles.css

@@ -112,6 +112,9 @@ main {
   background: var(--surface);
   border-radius: var(--radius);
   border: 1px solid var(--border);
+  width: 100%;
+  max-width: 525px;
+  contain: layout style;
 }
 
 .deck-selector {
@@ -155,6 +158,8 @@ main {
   align-items: center;
   justify-content: center;
   width: 100%;
+  min-width: 0;
+  min-height: 0;
 }
 
 .key-btn:hover {
@@ -169,11 +174,10 @@ main {
   width: 100%;
   height: 100%;
   object-fit: cover;
-  border-radius: inherit;
 }
 
 .key-btn.empty {
-  opacity: 0.3;
+  border-color: var(--border-light);
 }
 .key-btn.empty:hover {
   opacity: 0.5;