Dynamic key generators: plugin system for auto-generated pages

Add dynamic_keys support to PageConfig — a contract-based plugin
system where any executable outputs a JSON array of KeyConfig to stdout.
The daemon runs the generator on a configurable interval and renders
the keys dynamically on the Stream Deck.

Core changes:
- config: DynamicKeyGen struct (command/script, interval, timeout, max_keys)
- config: full validation for dynamic_keys fields
- action: execDynamicKeys() with safe JSON parsing (stderr logged separately,
  env vars STREAMDECK_KEY_COUNT / STREAMDECK_CONFIG_DIR)
- page: activeKeys() merges static + dynamic keys (static wins by index)
- page: startDynamicKeys / stopDynamicKeys lifecycle, per-page results cache
- page: rerenderActivePage() with deadlock-safe lock ordering
- daemon: event handler uses activeKeys() for dynamic key actions

Web UI:
- GET /api/pages returns effective_keys (merged static + cached dynamic)
- app.js: gridKeys uses effective_keys when available
- app.js: guard against editing dynamic keys through the UI
- index.html: "⚡ dynamic" badge + effective key count in page switcher
- polls /api/pages every 10s to keep the UI in sync

Image rendering:
- loadImage: gift.ResizeToFill instead of gift.Resize (preserves aspect
  ratio, crops to fill — no more stretched cover art)

Example & docs:
- config.example.json: games page with dynamic_keys + back button
- README.md: comprehensive Dynamic Key Generators section (architecture
  diagram, config reference, generator contract, merge behavior,
  env vars, error handling, Python + Shell examples, tips)

static/index.html

@@ -94,8 +94,11 @@
           :style="{ borderColor: p.name === activePage ? 'var(--accent)' : '' }"
           @click="switchPage(p.name); subView = null">
           <span x-text="p.name"></span>
+          <span x-show="p.dynamic_keys"
+            title="Dynamic keys generator"
+            style="color:var(--accent);font-size:11px;margin-left:4px;">⚡</span>
           <span style="float:right;color:var(--text-muted);font-size:11px;"
-            x-text="p.keys.length + ' keys'"></span>
+            x-text="(p.effective_keys?.length || p.keys.length) + ' keys'"></span>
         </button>
       </template>
     </div>