brand: rename product Mike -> Helix Tribune (user-facing strings only)

Identifiers (mikeApi, MikeChat, MikeIcon, MikeMessage, package names,
backend/db/storage docstrings) intentionally still reference "mike" -
that's the upstream project name and renaming them across the codebase
would be churn without benefit. This commit renames only what an end
user can see:

- Page <title>, OpenGraph, Twitter card, and metadataBase URL in
  frontend/src/app/layout.tsx
- Global error <title>, sidebar header, site-logo text + landing href
- Tabular and workflow column-prompt placeholders, model-settings
  copy, support form "question" description
- LLM system prompts ("You are Mike, ...") in chatTools.ts and
  routes/tabular.ts so the assistant introduces itself correctly
- DOCX tracked-changes author label so edited documents show
  "Helix Tribune" as the change author in Word
- Backend startup log line
- README heading + product description + website link
- CLAUDE.md product description (with a note that identifiers stay)

Domain refs `mikeoss.com` / `app.mikeoss.com` are replaced with
`tribune.helix.levelfive.studio` everywhere they're user-facing. The
upstream-repo references (`mikeoss/mike`, `willchen96/mike`) in
infra/UPSTREAM.md and the README's "fork of" line are deliberately
left alone - they correctly point at the upstream we forked from.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>