feat(device-sync): manifest, cancel, font picker + sync status fix

- Write psysonic-sync.json to device after sync/deletions; auto-import
  on folder select when localStorage is empty (cross-platform handoff)
- Add cancel button during active sync: AtomicBool flag per job,
  tasks bail after acquiring semaphore, cancelled state in UI
- Fix sync status staying "pending": normalize template path separators
  to OS separator (Windows: '/' → '\') so compute_sync_paths and
  list_device_dir_files produce matching strings for Set comparison
- Add Geist and JetBrains Mono as variable fonts; font picker collapsible
- Fix device mount detection on Windows: removable drive letters (E:\)
  were incorrectly skipped alongside system roots; strip \?\ prefix
  from canonicalized paths before mount-point comparison

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package-lock.json

@@ -10,7 +10,9 @@
       "dependencies": {
         "@fontsource-variable/dm-sans": "^5.2.8",
         "@fontsource-variable/figtree": "^5.2.10",
+        "@fontsource-variable/geist": "^5.2.8",
         "@fontsource-variable/inter": "^5.2.8",
+        "@fontsource-variable/jetbrains-mono": "^5.2.8",
         "@fontsource-variable/lexend": "^5.2.11",
         "@fontsource-variable/manrope": "^5.2.8",
         "@fontsource-variable/nunito": "^5.2.7",
@@ -803,6 +805,15 @@
         "url": "https://github.com/sponsors/ayuhito"
       }
     },
+    "node_modules/@fontsource-variable/geist": {
+      "version": "5.2.8",
+      "resolved": "https://registry.npmjs.org/@fontsource-variable/geist/-/geist-5.2.8.tgz",
+      "integrity": "sha512-cJ6m9e+8MQ5dCYJsLylfZrgBh6KkG4bOLckB35Tr9J/EqdkEM6QllH5PxqP1dhTvFup+HtMRPuz9xOjxXJggxw==",
+      "license": "OFL-1.1",
+      "funding": {
+        "url": "https://github.com/sponsors/ayuhito"
+      }
+    },
     "node_modules/@fontsource-variable/inter": {
       "version": "5.2.8",
       "resolved": "https://registry.npmjs.org/@fontsource-variable/inter/-/inter-5.2.8.tgz",
@@ -812,6 +823,15 @@
         "url": "https://github.com/sponsors/ayuhito"
       }
     },
+    "node_modules/@fontsource-variable/jetbrains-mono": {
+      "version": "5.2.8",
+      "resolved": "https://registry.npmjs.org/@fontsource-variable/jetbrains-mono/-/jetbrains-mono-5.2.8.tgz",
+      "integrity": "sha512-WBA9elru6Jdp5df2mES55wuOO0WIrn3kpXnI4+W2ek5u3ZgLS9XS4gmIlcQhiZOWEKl95meYdvK7xI+ETLCq/Q==",
+      "license": "OFL-1.1",
+      "funding": {
+        "url": "https://github.com/sponsors/ayuhito"
+      }
+    },
     "node_modules/@fontsource-variable/lexend": {
       "version": "5.2.11",
       "resolved": "https://registry.npmjs.org/@fontsource-variable/lexend/-/lexend-5.2.11.tgz",