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/app.js

@@ -52,6 +52,7 @@ document.addEventListener('alpine:init', () => {
       await this.loadConfig()
       await this.loadDecks()
       setInterval(() => this.pollDisplayOutputs(), 3000)
+      setInterval(() => this.loadEffectiveKeys(), 10000)
       this.connectSSE()
     },
 
@@ -101,6 +102,7 @@ document.addEventListener('alpine:init', () => {
         const res = await fetch('/api/config')
         this.config = await res.json()
         this.pages = this.config.pages
+        await this.loadEffectiveKeys()
         if (this.pages.length > 0) {
           const saved = localStorage.getItem('sd_active_page')
           if (saved && this.pages.find(p => p.name === saved)) {
@@ -115,6 +117,19 @@ document.addEventListener('alpine:init', () => {
       }
     },
 
+    async loadEffectiveKeys() {
+      try {
+        const res = await fetch('/api/pages')
+        const pagesData = await res.json()
+        for (const pd of pagesData) {
+          const page = this.pages.find(p => p.name === pd.name)
+          if (page && pd.effective_keys && pd.effective_keys.length > 0) {
+            page.effective_keys = pd.effective_keys
+          }
+        }
+      } catch (_) {}
+    },
+
     syncSettings() {
       this.settingsForm = {
         brightness: this.getDeviceConfig(this.activeDeckSerial)?.brightness ?? 75,
@@ -151,10 +166,13 @@ document.addEventListener('alpine:init', () => {
     get gridKeys() {
       const page = this.currentPage
       if (!page) return []
+      const sourceKeys = (page.effective_keys && page.effective_keys.length > 0)
+        ? page.effective_keys
+        : (page.keys || [])
       const keys = []
       const n = this.activeDeck.num_keys
       for (let i = 0; i < n; i++) {
-        const kc = page.keys.find(k => k.index === i)
+        const kc = sourceKeys.find(k => k.index === i)
         const dout = this.displayOutputs[i]
         keys.push({
           index: i,
@@ -166,6 +184,7 @@ document.addEventListener('alpine:init', () => {
           displayBg: dout?.background || '',
           displayText: dout?.text || '',
           previewUrl: this.keyPreviewUrl(kc ? kc : {}, dout),
+          isDynamic: !!(page.dynamic_keys && page.effective_keys?.find(k => k.index === i)),
         })
       }
       return keys
@@ -306,6 +325,9 @@ document.addEventListener('alpine:init', () => {
       const page = this.currentPage
       if (!page) return
       const kc = page.keys.find(k => k.index === idx)
+      if (!kc && page.dynamic_keys && page.effective_keys?.find(k => k.index === idx)) {
+        return
+      }
       this.editing = idx
       this.showAdvanced = false