Add pause/resume, within-part byte-range resume; bump to 0.0.3

Split the single morphing button into a green/amber play-pause (Update →
Pause → Resume) and a red stop (Cancel). Pause aborts the download but
keeps the partial (resumable); Cancel aborts and deletes. Backend gets a
separate pause flag distinct from cancel.

Resume now continues WITHIN a part: the partially-written piece is fetched
from its last byte via an HTTP byte range (with a safe fall back to
re-fetching the whole piece if the CDN ignores the range), instead of
re-downloading the whole part. A title is resumable as soon as any bytes
are on disk.

Version bumped to 0.0.3 (no release tagged).

web/index.html

@@ -39,7 +39,7 @@
           <div class="brand-mark">PD</div>
           <div>
             <strong>PatchDL</strong>
-            <span>by Knutwurst · v0.0.2</span>
+            <span>by Knutwurst · v0.0.3</span>
           </div>
         </div>