Add Thunderbird Companion plugin

[mdj2812]

Plugin

• Id: mdj2812/thunderbird-companion
• New plugin
• Update to an existing plugin (version bumped in plugin.toml)

What it does

Adds a Thunderbird bar widget, recent-unread panel, and background service without duplicating IMAP/SMTP credentials. A bundled, readable MailExtension and native host expose unread metadata plus open, reply, compose, mark-read, mark-all-read, and archive actions through local native messaging.

Setup builds the XPI locally, installs the native host under XDG data paths, and leaves Thunderbird's permission approval as an explicit manual step. Cached messages remain visible while the native-host heartbeat distinguishes live and offline state.

External dependencies

• thunderbird 128 or newer: mailbox, compose UI, and MailExtension APIs.
• python3: bundled native-messaging host and deterministic local XPI build.
• xdg-open: manually opens the generated XPI directory from the setup panel.

Testing

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Noctalia version tested against: noctalia v5.0.0 (v5.0.0-beta.8-50-g713e6caa3ac6)
• Plugin API level: 9

Screenshots / Videos

Checklist

• The directory name matches the part of id after the / in plugin.toml exactly.
• It ships plugin.toml, README.md, thumbnail.webp, and translations/en.json.
• README.md follows the
  README template, documents
  every entry id and dependency, and includes exact panel IPC commands and launcher prefixes where applicable.
• I created thumbnail.webp with the thumbnail generator.
• version follows semver and is bumped in this PR; plugin_api is the oldest API level this plugin requires.
• Every non-English translation in this PR uses a locale supported by Noctalia core, and I can read, write, and
  understand that language well enough to review and maintain it (no unreviewed machine/LLM translations).
• I did not edit catalog.toml; CI generates it.
• This PR touches exactly one plugin directory.

Code review attestation

Plugins run as trusted, unsandboxed Luau in the user's session. Confirm:

• The code is readable and not obfuscated, minified, or generated.
• It does not download and execute remote code.
• Every network call, filesystem write, and spawned process is something the description above accounts for.
• I have the right to publish this code under the license declared in plugin.toml.

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