feat: add paginated full views for user PRs and commits

[jordangarrison]

Summary

• Add /:username/pulls and /:username/commits routes with paginated full-view LiveViews (10 items per page)
• Increase UserInsightsServer cache from 5 to 50 items with optional per_page parameter flowing through Ash domain, actions, and GitHub client
• Add "View all →" links to dashboard PR and commit section headers, limit dashboard preview to 5 items

Changes

Backend

• GitHub Client: search_user_prs/2 and search_user_commits/2 accept optional per_page via opts map (default 50)
• Ash Resources: Added optional per_page argument to UserPR and UserCommit list actions
• ManualRead Actions: Extract per_page from query arguments, pass through to client (follows ListWorkflowRuns pattern)
• UserInsightsServer: Fetches 50 items per poll cycle instead of 5

Frontend

• Dashboard: "View all →" links on section headers, Enum.take(5) for preview cards
• UserPullsLive: Full paginated view at /:username/pulls with ?page=N query params
• UserCommitsLive: Full paginated view at /:username/commits with same pagination pattern
• Both views read from cache, subscribe for live updates, use handle_params for back/forward navigation

Test Plan

• 73 tests passing (10 new tests for the two LiveViews)
• mix precommit passes (compile, format, credo, tests)
• Nix build succeeds
• Manual: visit dashboard, click "View all →" links
• Manual: verify pagination controls on /username/pulls and /username/commits
• Manual: verify back/forward browser navigation between pages