appview/pages: fix concurrent write in dev mode

When in dev mode. Concurrent writes to the template map will cause a
concurrent write panic and crash appview. Quickly moving between pages
can trigger this crash. This is addressed by adding a RWMutex on the
templates. RWMutex allows multiple readers, but only one writer.

Signed-off-by: Ciel <[e-mail]>

appview/pages/pages.go

···
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 
 	"tangled.sh/tangled.sh/core/appview/commitverify"
 	"tangled.sh/tangled.sh/core/appview/config"
···
 var Files embed.FS
 
 type Pages struct {
-	t           map[string]*template.Template
+	mu sync.RWMutex
+	t  map[string]*template.Template
+
 	avatar      config.AvatarConfig
 	dev         bool
 	embedFS     embed.FS
···
 	}
 
 	p := &Pages{
+		mu:          sync.RWMutex{},
 		t:           make(map[string]*template.Template),
 		dev:         config.Core.Dev,
 		avatar:      config.Avatar,
···
 	}
 
 	log.Printf("total templates loaded: %d", len(templates))
+	p.mu.Lock()
+	defer p.mu.Unlock()
 	p.t = templates
 }
 
···
 	}
 
 	// Update the template in the map
+	p.mu.Lock()
+	defer p.mu.Unlock()
 	p.t[name] = tmpl
 	log.Printf("template reloaded from disk: %s", name)
 	return nil
···
 		}
 	}
 
+	p.mu.RLock()
+	defer p.mu.RUnlock()
 	tmpl, exists := p.t[templateName]
 	if !exists {
 		return fmt.Errorf("template not found: %s", templateName)