feat(settings): add home content customization and config injection

- Add home_content setting for custom homepage (HTML or iframe URL)
- Inject public settings into index.html to eliminate page flash
- Support ETag caching with automatic invalidation on settings update
- Add Vite plugin for dev mode settings injection
- Refactor HomeView to use appStore instead of local API calls

backend/internal/web/html_cache.go

@@ -0,0 +1,77 @@
+//go:build embed
+
+package web
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"sync"
+)
+
+// HTMLCache manages the cached index.html with injected settings
+type HTMLCache struct {
+	mu              sync.RWMutex
+	cachedHTML      []byte
+	etag            string
+	baseHTMLHash    string // Hash of the original index.html (immutable after build)
+	settingsVersion uint64 // Incremented when settings change
+}
+
+// CachedHTML represents the cache state
+type CachedHTML struct {
+	Content []byte
+	ETag    string
+}
+
+// NewHTMLCache creates a new HTML cache instance
+func NewHTMLCache() *HTMLCache {
+	return &HTMLCache{}
+}
+
+// SetBaseHTML initializes the cache with the base HTML template
+func (c *HTMLCache) SetBaseHTML(baseHTML []byte) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	hash := sha256.Sum256(baseHTML)
+	c.baseHTMLHash = hex.EncodeToString(hash[:8]) // First 8 bytes for brevity
+}
+
+// Invalidate marks the cache as stale
+func (c *HTMLCache) Invalidate() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.settingsVersion++
+	c.cachedHTML = nil
+	c.etag = ""
+}
+
+// Get returns the cached HTML or nil if cache is stale
+func (c *HTMLCache) Get() *CachedHTML {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	if c.cachedHTML == nil {
+		return nil
+	}
+	return &CachedHTML{
+		Content: c.cachedHTML,
+		ETag:    c.etag,
+	}
+}
+
+// Set updates the cache with new rendered HTML
+func (c *HTMLCache) Set(html []byte, settingsJSON []byte) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.cachedHTML = html
+	c.etag = c.generateETag(settingsJSON)
+}
+
+// generateETag creates an ETag from base HTML hash + settings hash
+func (c *HTMLCache) generateETag(settingsJSON []byte) string {
+	settingsHash := sha256.Sum256(settingsJSON)
+	return `"` + c.baseHTMLHash + "-" + hex.EncodeToString(settingsHash[:8]) + `"`
+}