Download refactor (UI & Logic)

[ItsDark008]

What

• resume partial downloads from servers that do not send an ETag, and only
  resume when the response is a matching 206 Partial Content
• keep the stored ETag across resumed responses and clear it when a download
  restarts from scratch, so a later resume is not rejected by a stale validator
• add SidebarDownloadStatus with progress, speed and phase for the active job,
  plus resume, pause and cancel quick actions
• collapse the sidebar item to a compact progress indicator that opens the
  downloads section
• extract ProgressBar and CancelDownloadDialog and reuse them in
  DownloadsView
• poll download progress from the store at app level instead of once per view
• keep HomeView rendered while downloads refresh in the background

Why

Interrupted downloads restarted from zero whenever the server omitted ETag,
because a stored validator was required to resume. The sidebar also gave no
feedback about a running download unless the downloads page was open.

Review notes

• polling moved from DownloadsView into stores/downloads.ts and starts during
  bootstrap, so it now runs on every section while a download is active
• new Rust tests cover resume without ETag, resume with a changed ETag and the
  cleared validator after a restart

[ItsDark008]

Superseded by #96.

• the two Rust commits here (ea9d1ad, 9f2eb4b) already landed on main through fix: Fix download resume #93 (a2bc1e0)
• the frontend commit is now feat: add sidebar download status with resume, pause and cancel actions #96.