feat(learning): roadmap completion screen with share post and next-roadmap flow

[OthmaneZ05]

Summary of Changes

Finishing a roadmap now opens a completion screen over the canvas instead of a plain toast. It celebrates the run with facts read off the real play-through and turns it into something the learner can carry out of the app:

• Run receipt (single mono block, copyable): roadmap id, project, steps passed, started/finished timestamps. The progress store now stamps startedAt when a play-through's entry is created (backend + docs/learning-api.md updated).
• Steps recap in two columns and skills chips derived from the validators — same deriveTopology source as the briefing page, no parallel derivation.
• Downloadable share image card (1200x630): renders the run receipt as a PNG, sized to its content, with its own legible font scale and DPR handling; the on-screen preview is decoupled from the downloaded image size and scales to its container width.
• Copy share post (primary action): copies the share text together with the share image in one ClipboardItem, EN/FR.
• Next roadmap: resolves the first unstarted (then first unfinished) roadmap of the catalogue in the UI language and lands on its briefing page in the home shell — new LearningExit intent mirroring LearningIntent, consumed one-shot by ProjectsPage. Falls back to "All roadmaps" plus a catalogue-complete note when nothing is left.
• Keep building link dismisses back to the canvas (containers keep running); the player then shows a quiet "Roadmap complete — View summary" row so the screen stays replayable, and reopening an already-finished roadmap replays it.
• The completion detection lives in useLearningPlayer (fresh completion = the verdict that turned the last missing step green; deduplicated so re-validating a complete roadmap never reopens the screen). The step toast is suppressed/consumed when the celebration opens.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass)

Backend: npm run build clean, 448/448 tests pass. Frontend: npm run build clean, lint clean (0 errors, 6 pre-existing warnings unrelated to this change), 299 tests pass — 2 initially timed out under parallel load (CanvasPage rename test, PostgresModal tabs test, both unrelated to this feature) and passed cleanly re-run in isolation.

Manual Verification

Playwright recipe against the dev server with every backend call mocked via page.route() (progress = all steps but the last passed), run in EN and FR:

• Validate on the last step → completion screen opens with the receipt (started/finished from the store), steps recap, skills chips, and the share image card preview.
• "Copy share post" → clipboard holds the share text and the share image together as a single ClipboardItem.
• "Keep building" → back on the canvas with no stale step toast; "View summary" reopens the screen.
• "Next roadmap" → home shell lands directly on the next unfinished roadmap's briefing page.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing