feat: add board filter to the Issues page

[nyg]

Adds a Boards filter to the Issues sidebar.

• Each issue now carries the open GitHub Projects boards it belongs to, shown as owner/title.
• The Boards section sits between Repositories and Labels, with the same Include only / Exclude toggle as Authors.
• If the token cannot read Projects, the issue query is retried without board fields, so the Issues page still loads and the Boards section is hidden.
• A saved board selection is ignored while the section is hidden.
• The board selection and mode are saved with the other sidebar filters.
• New sidebar labels are translated in all six languages.