chore(web): comments say why, never what; long ones move to docs (CL-8378)

[TheGreatAxios]

Summary

• Every comment block of 4+ consecutive lines under apps/web/src (excluding settings/inference/** and onboarding/**, owned by another in-flight branch) was reviewed: deleted if it narrated what the code does, shortened to its why (1-3 lines) if it held one, or moved into a short docs/*.md section with a one-line pointer left in code when it held real rationale worth preserving.
• No behavior changes — comment-only edits, verified with bun run typecheck after every batch.
• 4+-line comment blocks in scope: 537 before, 0 after.

Changes

• New docs: docs/chat-wire-contract.md, docs/chat-mail-threading.md, docs/settings-sections.md, docs/routines-scheduling.md, docs/test-toast-mock.md, docs/myra-definition-json.md, docs/insights-native-runs.md, docs/auth-dither-background.md.
• Trimmed comments across chat, shell, settings, pages, library, lib, command-palette, and the top-level app/api/routing modules.

Testing

• bun run typecheck (repo root) — passes
• bun run lint — passes (pre-existing unrelated warnings in apps/sidecar)
• bun run fmt:check — passes
• bun run test (repo root, includes apps/web's build + bun test ./src) — 266 + 621 pass, 0 fail

[linear-code]

CL-8378