Issue Backlog View tree view of squad-labeled GitHub issues

## Description
Add a new tree view showing all `squad:{member}` labeled issues from GitHub, grouped by member and priority. Shifts SquadUI from a read-only observer to an active project management tool.

## Approach
- New `SquadBacklogTreeProvider` tree data provider
- Group by member (squad:{member} labels), then by priority label
- Show issue title, number, state, and assignee
- Click to open issue in browser or show details in a webview
- Requires GitHub API pagination + caching for rate limits
- Extends existing `GitHubIssuesService`

## Acceptance Criteria
- [ ] New 'Backlog' tree view appears in the Squad sidebar
- [ ] Issues grouped by squad member
- [ ] Priority sub-grouping within each member
- [ ] Click opens issue details
- [ ] Refresh button to re-fetch from GitHub
- [ ] Handles rate limiting gracefully