Show a label's description when hovering its chip in pull requests

[TonybynMp4]

Problem

Label names are often shorthand: vouch:unvouched, size:S, macroscope-review. GitHub lets repos attach a description to each label and shows it on hover, but T3 Code's PR list and PR detail chips only show the name. To learn what size:S means you have to leave the app.

The label picker already fetches and shows descriptions, so the data is there for repos that set them. The chips just don't get it.

Proposal

Hovering a label chip shows its description in the existing tooltip. Labels with no description behave exactly as today, with no tooltip.

Scope

• Contracts: PullRequestLabel gets an optional description. It's optional so a newer client talking to an older server still decodes.
• GitHub: request description next to name and color in the listing and detail queries.
• Forgejo: already decodes description, so it's mapped through.
• GitLab, Bitbucket, Azure DevOps: their PR/MR payloads carry label names only (or no descriptions), so no tooltip there. GitCafe is unchanged for now; its decoder only reads name and color.
• Web: PullRequestLabelChip wraps itself in a tooltip when a description exists. That covers the list rows, the detail view and the loading ghosts.
• Mobile: left out, since there's no hover. Long-press could come later if wanted.

Payload cost is one short string per label on the list query (capped at 20 labels per PR already).

I have a working branch with this (about 50 lines plus regenerated GraphQL types) and can open a PR if the direction looks fine.