feat: add apt-manager extension

extensions/apt-manager/.gitignore

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+node_modules
+vicinae-env.d.ts
+AGENT.md
+.agent

extensions/apt-manager/README.md

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+# apt-manager
+
+Manage apt packages and repositories from [Vicinae](https://docs.vicinae.com).
+
+## Features
+
+Launched from the `apt` command:
+
+- **Installed packages** — list what's installed, remove/reinstall, show info.
+- **All packages** — browse the full apt cache (lazy-loaded), install on the fly.
+- **Upgradable packages** — see updates, upgrade individual packages.
+- **Batch operations** — in the All / Installed / Upgradable lists, pick
+  "Select multiple packages…" to choose several packages and install, remove,
+  or upgrade them in a single `apt-get` command.
+- **Update all** — `apt update` then `apt-get upgrade --with-new-pkgs`, with confirmation.
+- **Clean up system** — `apt-get autoremove --purge` then `apt-get autoclean`.
+- **Repositories** — list sources from `sources.list`/`sources.list.d` (legacy one-line
+  and deb822 `.sources`), add new repos as deb822 `.sources` files, enable/disable/remove.
+- **Install package files** — install a local `.deb` (via `apt`), `.flatpak`
+  (via `flatpak`), or `.AppImage` file from one dialog. The extension detects
+  the extension and runs the matching method.
+- **AppImages** — install (from a local file) and remove AppImage applications
+  installed in `~/Applications`. Download an AppImage yourself, then point the
+  extension at the file; it copies it into `~/Applications` and, when
+  `unsquashfs` (package `squashfs-tools`) is available, extracts its icon and
+  metadata for the launcher entry. The AppImage is **never executed** during
+  installation. Without `squashfs-tools`, the launcher entry uses the filename.
+
+## How it works
+
+- Read-only operations (`apt list`, `apt-cache show`) run without privileges.
+- Every privileged operation (`install`, `remove`, `upgrade`, `cleanup`)
+    is run via `pkexec`, which shows the desktop's polkit authentication dialog.
+    If `pkexec` is missing you'll get a readable error plus a "Retry in Terminal (sudo)"
+    action instead. Repository writes are also privileged but do not provide a terminal retry fallback; use `sudo tee` or your editor instead.
+- Command output for long-running operations is shown in a result view after completion.
+
+## Development
+
+```bash
+npm install
+npm run dev        # run in development mode (requires vicinae running)
+npm run build      # production bundle
+npm run lint
+```
+
+## Requirements
+
+- Debian / Ubuntu (or a resolvable) system with `apt`.
+- `pkexec` (package `policykit-1`) for privileged operations — either the
+  polkit agent of your desktop session or the fallback terminal action.
+- Optional: `flatpak` to manage Flatpak applications and repositories. The
+  Flatpak sections are hidden when the CLI is not installed.
+- Optional: `squashfs-tools` (`unsquashfs`) to extract AppImage icon and
+  metadata without running the file; without it AppImages are listed by their
+  filename.