Add plugin entry: file-manager

[xMinor-1]

What the plugin does

Adds a full-page file manager for the machine that runs the bb server, reachable
from a "File Manager" item in the sidebar. It covers the work that otherwise
needs a shell: a tree whose folders expand in place, drag-and-drop upload,
streamed download, move, copy, rename, delete, and archive extraction.

Uploads are chunked and resumable, so a dropped connection continues instead of
starting over — tested on a 5 GB file. Downloads are streamed rather than
buffered.

Source release

• git: https://github.com/xMinor-1/bb-plugins.git
• subdir: plugins/file-manager
• tagPrefix: file-manager/, range: ^0.3.0
• Released tag: file-manager/v0.3.0 → commit 54eddc767af63a0183d0949c59ee59f83e2b3c61

Plugin checks

Run against the exact released commit in a clean worktree:

• npx tsc --noEmit — clean
• bb plugin build . — builds dist/server.js, dist/app.js, dist/app.css and their meta
• Engines declared by the manifest: bb >=0.39, bbPluginSdk >=0.4.8

Marketplace checks

• npm ci --ignore-scripts, npm run build, npm run check — all pass, 64 entries
• Icon: the BB host icon name FolderOpen, the same one the plugin manifest declares

Permissions and security

This plugin reads and writes files on the bb host, which is its purpose, so the
boundary matters:

• Every path is resolved with realpath before the root prefix test, so a
  symlink cannot be used to address anything outside the hard root — the home
  directory of the user running bb. A path that resolves outside is refused
  (src/root.ts, which is the only place in the plugin allowed to build a
  filesystem path from user input).
• Archive extraction treats an archive as attacker-controlled data. Entries are
  staged into a private <dest>/.bb-extract-<hex>/out directory the plugin
  owns, so a ../ escape (the classic zip-slip) still lands inside it; anything
  written beside out is swept away; every symlink in the result is re-checked
  against the hard root once it sits at its final path; only then is the tree
  committed to the destination.
• Extraction does shell out to tar / unzip / 7z, via spawn with an
  argument list rather than a shell string, with --no-same-owner --no-same-permissions. The client cannot name a command or a destination —
  the format is detected by the plugin and the destination is derived from the
  validated path.
• All client operations go through the plugin's own authenticated RPC and HTTP
  routes.
• No network calls to anything outside the bb host.