feat(feed-directory): rebuild browse UI as layered, testable module tree

[gildesmarais]

What changed

• Replaced monolithic FeedDirectory.astro, feed-directory.js, and catalogClient.js with src/components/feed-directory/ (adapters, domain, app, ui, CSS) mounted from feed-directory/FeedDirectory.astro.
• Catalog wire handling lives in adapters/catalog-api.ts: GET /api/v1/configs envelope validation, row normalization to FeedDirectoryEntry, and catalog_version gate (supported: [1]).
• Browser concerns split into adapters (browser-storage, browser-location, browser-download) so domain code stays free of window / document.
• App layer uses directory-state.ts for filter/pagination transitions and FeedDirectoryApp.ts for fetch/render orchestration; ui/render.ts builds HTML from a view model.
• Added Vitest (vitest.config.ts, make test, make check) with unit tests for catalog parsing, filters, language matching, OPML, and directory state.
• UX: two-column feed cards, URL-synced filters (q, topic, lang, sort, page), base language codes (de matches de-DE), disabled Starlight TOC on /feed-directory/ for full width.
• Docs: CONTEXT.md glossary (catalog seam, instance persistence), expanded AGENTS.md module layout and verification gates; removed legacy instanceUrl.js.

Why

The prior browse UI was hard to scan and lived in one large script bundle. The rebuild keeps the same thin-client contract over a running html2rss-web instance while making layout, filtering, and instance deep links maintainable. Follow-on commits after the initial tree split wire parsing and browser I/O behind adapters and add tests so catalog envelope changes fail in CI instead of only in the browser.

Risk

• Low functional risk — still client-only over GET /api/v1/configs; instance default, #!url= deep link, and html2rss.feedDirectory.instanceUrl localStorage behavior are documented in CONTEXT.md.
• Intentional break: no legacy DOM hooks or old script paths (feed-directory.js, catalogClient.js, instanceUrl.js).
• After deploy, spot-check filter URL sharing, instance change, OPML export, params rows, and catalog-disabled / unreachable instance error states.

Review map

1. src/components/feed-directory/adapters/catalog-api.test.ts and domain/filters.test.ts — expected wire shapes, filter/language edge cases
2. src/components/feed-directory/adapters/catalog-api.ts — sole catalog wire parser; version gate and FeedDirectoryEntry mapping
3. src/components/feed-directory/app/FeedDirectoryApp.ts and app/directory-state.ts — load/error flows and state transitions vs URL params
4. src/components/feed-directory/adapters/browser-*.ts — instance persistence, filter sync, OPML download
5. src/components/feed-directory/ui/render.ts and feed-directory.css — row layout and Starlight-aligned styling
6. CONTEXT.md and AGENTS.md — glossary and contributor contracts

Validation

• make check (lint + Vitest, exit 0)
• make build (exit 0 on prior PR pass; re-run before merge if Astro content changed since)
• Manual: spot-check /feed-directory/ against an instance with catalog enabled