Fix appVersion IPC, ICS/vCard spec nits, and add ESLint to CI

[tomcardoso]

Summary

Batches three pre-1.0 audit fixes:

• window.sourcerer.appVersion is likely empty in packaged builds (broken version link in Settings) #432 — window.sourcerer.appVersion relied on process.env.npm_package_version, which is only set when launched via an npm script, so it's empty in packaged builds (broken version link + 404 on GitHub in Settings). Now exposed from the main process via app.getVersion() over a synchronous IPC call (app:get-version), matching the pattern the About panel already uses. Not verified against an actual packaged build — that check is called out as needing a human pass.
• Text/format spec nits: ICS & vCard line folding, vCard TYPE sanitization, astral HTML entities in headlines #440 — three small spec-compliance nits:
  • generateIcal / buildVCard now fold content lines at 75 octets per RFC 5545 / RFC 6350, with continuation lines prefixed by a single space and folds guaranteed not to split multi-byte UTF-8 sequences (new shared foldLine helper in src/main/utils.ts, covered by src/test/fold-line.test.ts).
  • buildVCard's EMAIL;TYPE=... parameter is now sanitized to alphanumerics (falling back to INTERNET) instead of interpolating the raw, user-editable email label.
  • cleanHeadline now decodes numeric/hex HTML entities with String.fromCodePoint instead of String.fromCharCode, so astral codepoints (e.g. emoji in RSS/Google Alerts headlines) decode correctly instead of producing garbage surrogate halves.
• Add ESLint to CI #443 — added a flat ESLint config (typescript-eslint recommended + eslint-plugin-react-hooks's rules-of-hooks/exhaustive-deps, split Node vs browser globals for src/main+src/preload+src/test vs src/renderer), a lint script, and wired it into ci.yml and preversion. Scripts/build config outside the TS project (scripts/, build/, electron-builder.config.js) are excluded, matching what's already typechecked.

Turning the linter on surfaced a real bug: in ProjectTab.tsx, useListboxKeyboard was called after an early if (!membership) return null;, so the hook was skipped whenever a contact had no project memberships — a genuine conditional-hook violation. Moved the hook (and the option-filtering it depends on) above the early return. Also fixed a handful of mechanical findings (an unused let, a dead assignment, an unnecessary regex escape, unused type imports, a stale/unused test mock) and left the remaining exhaustive-deps findings as warnings, since they don't block CI and each would need individual judgement calls about effect timing.

Test plan

• npm run typecheck passes
• npm run lint passes (0 errors, 7 pre-existing exhaustive-deps warnings, non-blocking)
• npm test — 490/490 passing, including new foldLine coverage
• Verify appVersion renders correctly in an installed/packaged build (not verifiable from source)

Closes #432
Closes #440
Closes #443

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