feat(deep-link): handle autonomi:// URLs to trigger downloads

[Nic-dorman]

Why

Clicking an autonomi://<public_address> link in a browser should launch/focus the desktop app and start downloading that file — the public address is already the shareable handle, so a clickable link is the natural next step. Public-only by design (a private upload's datamap isn't URL-shareable).

What

• Registers the autonomi:// URL scheme and routes a clicked link into the existing Download by address flow: the dialog opens prefilled with the address, which doubles as the confirm + filename step (a web-triggered link never downloads silently).
• Optional filename via autonomi://<addr>?name=<file>, prefilled into the dialog (validated by the existing filenameError check).

How

• Plugins: tauri-plugin-deep-link (scheme declared in tauri.conf.json) and, on Windows/Linux, tauri-plugin-single-instance with the deep-link feature so a link opened while the app is already running routes into the existing window instead of spawning a duplicate. macOS delivers to the running app via on_open_url.
• src-tauri/src/lib.rs: register single-instance first (Win/Linux), init deep-link, register_all() at runtime for dev, capture cold-start URLs into DeepLinkState (drained via take_pending_deep_links), and emit a deep-link event for warm opens. On Win/Linux the single-instance callback also parses the autonomi:// URL out of argv and emits it, so warm-start doesn't depend solely on feature forwarding.
• Frontend (plugins/deep-link.client.ts): validates the URL (parseAutonomiDeepLink / isValidPublicAddress), stashes the address (+ optional name) on the files store, and routes to /files. The store + router are captured at plugin init because the event callback runs outside the Nuxt instance context. DownloadDialog applies the prefill on mount (not just on open-transition) so it works on the very first cold launch, and refreshes if a link arrives while it's open.

Testing

• Verified end-to-end on Windows (warm start, including first attempt after a fresh launch): clicking the link focuses the app, opens the Download dialog prefilled with address + ?name=, and download_public fetches/decrypts/saves.
• nuxi typecheck clean, vitest 37/37 (adds parseAutonomiDeepLink / isValidPublicAddress unit tests), cargo check clean.
• Still to validate: true cold start (app fully closed → click link). This can't be exercised under tauri dev (the bare dev exe has no Vite server), so it needs an installed/bundled build — pending verification on macOS.

Out of scope

• Private/datamap downloads (not URL-shareable).
• Per-row single export and other unrelated flows.

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