fix(web): derive share topics from evidence tags

[darron]

Replace public chat share topic generation with an evidence-weighted ranking built from research-pack user tags. Prioritize tags from sources cited in the answer, then explicit citations, then coverage tags, and run all labels through the configured category vocabulary.

This removes the old keyword/source-type guessing that produced generic chips such as research, media, software, GitHub, or YouTube. Public shares now show up to four specific topics and may show fewer or none when the pack has no useful tags.

Risk: topic quality now depends more directly on source tagging and vocabulary coverage. Follow up by watching real shared pages for missing or over-specific topics and tuning the vocabulary/drop list as needed.

Add regression coverage for citation weighting, coverage fallback, vocabulary cleanup, and public-page chip rendering. Regenerated UI build artifacts are included with the commit.

[github-actions]

PR Diff Stats

PR #51 changed 3 file(s), with +261 / -45 (306 changed lines).

Bucket	Files	Additions	Deletions	Changed
Docs	1	+10	-0	10
Tests	1	+103	-2	105
Migrations	0	+0	-0	0
Generated	0	+0	-0	0
Lockfiles	0	+0	-0	0
Config / Ops	0	+0	-0	0
Code	1	+148	-43	191
Total	3	+261	-45	306

Largest Changed Files

File	Bucket	Additions	Deletions	Changed
web/chat_shares.go	Code	+148	-43	191
web/chat_shares_test.go	Tests	+103	-2	105
CHANGELOG.md	Docs	+10	-0	10

Bucket rules

• Docs: Markdown/text docs such as docs/**, README.md, and other *.md or *.txt files
• Tests: *_test.go, test/, tests/, e2e/, and JS/TS *.test.* or *.spec.* files
• Migrations: schema migration/init files such as internal/store/migrations.go and internal/store/schema_init.go
• Generated: local runtime/generated surfaces such as data/, logs/, tmp/, vault/, and web/ui/dist/
• Lockfiles: dependency lockfiles such as go.sum and package-lock.json
• Config / Ops: CI, config, packaging, skills, deploy, and scripting files such as .github/, packaging/, skills/, scripts/, Makefile, Dockerfile, Taskfile.yml, config.yaml.sample, and fly.toml
• Code: everything else