feat(citations): open a citation chip at its source page and block

[mrsibe]

Related issue

Fixes #72. Follows #69/#70 (citation snapshot + resolution) and #71 (reader contract).

What changed?

• shared/utils/sourceAnchor.ts — citationToSourceAnchor, sourceAnchorToSearchParams, sourceAnchorFromSearchParams, withSourceAnchor, sourceAnchorsEqual. Pure: no DB, no DOM.
• shared/types/source.ts — SourceAnchor { documentId, location }. It is the "open this source and go there" unit; location stays a ReaderAnchor so it can go straight to openAt.
• chat/citationMarkers.ts — a remark plugin that rewrites [n] into a #citation-n link. It never decides presentation.
• chat/CitationChip.tsx — renders [n] title · p.5, disables when the source is gone, falls back to plain text for an unresolved marker.
• chat/MessageItem.tsx — maps markers to citations (parseCitations) and opens chips through the navigation hook.
• hooks/useSourceAnchorNavigation.ts — the single openSourceAnchor / closeSourceAnchor entry point (store + URL query). This is what [Feat] Selection → excerpt to Note, anchored back to its source #73 will reuse.
• SourcePanel.tsx — consumes the anchor, hydrates it from the URL on reload, passes it to the reader, clears a stale source query, and handles Esc.
• reader/anchor.ts + both readers — resolveAnchor() implements the location priority and clears a stale highlight.

Design notes

• One unit, two ends. ReaderAnchor answers "where inside the reader I already have"; SourceAnchor answers "which source, and where". [Feat] Citation click → open source at page/block with highlight #72 and [Feat] Selection → excerpt to Note, anchored back to its source #73 share the latter, not a bare ReaderAnchor.
• Location priority: blockId → startOffset/endOffset → page → document top. blockId is a derived index, so it is a fast hint, not the only truth — after a reindex the canonical offsets recover the block.
• Deep link is a query on /notebook/:id (?doc=&page=&start=&end=&block=), not location.state, so a reload restores the position. Parsing is defensive per field; closing the reader removes only those keys.
• Parsing ≠ presentation. The markdown only yields a marker link; the chip owns the label, the page suffix and the disabled state, so Citation never carries a display string.

How was this tested?

• npm run typecheck — passes.
• npm test — 190 tests pass, 17 new:
  • test/sourceAnchor.test.ts — citation → anchor mapping, decode(encode(anchor)) === anchor, per-field defensive parsing (page=abc, page=-1, end < start, lone end), withSourceAnchor preserving unrelated query keys.
  • test/citationMarkers.test.ts — [n] becomes a link, several markers in one paragraph, inline/fenced code and reference links are untouched.
  • test/sourceReader.test.ts — resolveAnchor priority, including stale block id recovered through offsets.
• npm run build — passes.
• npm run check:design — no violations.

Manual QA (needs a real Electron window — not verified headlessly)

• For the eval ground-truth question, the chip lands on the correct page and highlights the correct paragraph.
• Several citations in one answer work repeatedly, including two citations on different pages of the same document.
• A citation whose document was deleted is a disabled chip.
• An old deep link to a deleted source returns to the list and drops the stale query.
• Esc returns to the chat; the position restores after an app reload.
• Highlight overlay stays aligned with the page at 50% / 100% / 150% zoom (this depends on [Feat] Source Reader architecture — PDF first (replace the chunk-concatenation <pre> viewer) #71's geometry).

Screenshots / recordings

Not captured. A real-window pass is still required before merge; this PR does not claim the GUI behaviour is verified.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable
• npm run build passes.