feat: accept a .torrent dragged onto the search field

[ugurckr]

What and why

Dragging a file onto a terminal window pastes its path into whatever is reading input. That's the
gesture people already reach for when they have a .torrent on disk, and torlink half-supported
it: the search field took a magnet or a bare infohash, but a path was just a search query, so the
obvious move silently searched for C:\Users\...\thing.torrent and found nothing. torlnk <file>.torrent worked from the shell, but there was no way in from inside the running app.

The search field now recognises a .torrent path and downloads it. The work is in getting the
path back out of what the terminal actually pastes, because no two emulators agree:

Emulator	What lands in the field
Windows Terminal, PowerShell	"C:\Users\u\My Show.torrent" plus a trailing space
macOS Terminal, iTerm2	/Users/u/My\ Show\ \(2024\).torrent
GNOME Terminal	file:///home/u/My%20Show.torrent

resolveTorrentPath unwraps all three. The backslash rule is split by platform — an escape on
macOS and Linux, a path separator on Windows — because unescaping a Windows path would destroy it.
It takes the platform as an option rather than reading process.platform only, so the tests drive
both from either host instead of covering only the half that matches the runner.

Two smaller things follow from the same helper: paste (v) takes a path too, since copying a file
in a file manager is how a path gets on the clipboard in the first place; and torlnk <file>.torrent runs its argument through the same normalizer, so a shell-quoted or file://
argument works there as well.

A path that turns out not to be a readable torrent reports that, rather than falling back to a
search — the one thing worse than not supporting the gesture is supporting it silently badly.

No new key and no change to an existing one. ↵ on the search field already meant "do something
with this"; this widens what it accepts, next to the magnet and infohash cases that were already
there. The splash placeholder now says so.

Checklist

• npm run typecheck is clean
• npm test passes — with one caveat: on my Windows box 5 test files (daemon/runtime,
  daemon/serve, daemon/watch, download/queue, download/queue.safemode) fail to load
  node-datachannel. They fail identically on a clean main here, so it's my environment and
  not this change. Everything else passes, including the 15 new tests.
• New logic has a test — src/sources/torrentPath.test.ts, covering all three emulator
  shapes on both platforms, ~ expansion, uppercase extensions, and the cases that must stay
  a search (plain query, magnet, blank, a non-torrent file).
• No new key, so HELP_GROUPS / footerHints are unchanged
• No new Store field, so makeStore is unchanged
• OS-touching code works on Windows, macOS, and Linux — that's the substance of the change
• One concern, Conventional Commits title

Note on ordering

This is independent of my other PR (fix: keep a torrent's own trackers…) and the two touch
different files, so they merge in either order. They do compose, though: dropping a private
torrent's file is exactly the case where the discarded announce list mattered most.