feat: add Voyager-first capabilities and PieFed parity

[aeharding]

Summary

• expose exhaustive provider capabilities with request-aware support checks
• add provider-specific profile sorting and safer public type contracts
• implement Voyager-critical PieFed workflows, including notifications, reports, modlog, profile content, settings, logout, bans, image deletion, and site metadata
• harden malformed-response handling, version detection, and credential privacy
• expand stateful Lemmy and PieFed fakes plus real-API acceptance coverage
• pin the reviewed PieFed OpenAPI snapshot and enforce deterministic generation and drift checks
• strengthen CI, dependency maintenance, and testing documentation

Validation

• pnpm test — 656 passed, 48 skipped
• pnpm format:check
• pnpm build
• pnpm docs:build
• pnpm audit --audit-level moderate — no known vulnerabilities
• credential-free live acceptance — 53 passed, 1 intentional skip
• authenticated live Lemmy acceptance — 24 passed across all three configured disposable accounts
• exact packed commit 56c52e5 installed into Voyager 6a7999ea
  • typecheck, full ESLint with zero warnings, and full Prettier passed
  • 7 focused files / 48 tests passed
  • full unit suite: 25 files / 302 passed / 1 todo
  • production PWA build passed; 3,194 modules transformed and service worker generated

Notes

• the reviewed PieFed OpenAPI snapshot is committed intentionally so generated output is reproducible
• unsupported or lossy provider variants remain explicitly capability-gated rather than silently approximated