Make empty/loading/error copy lawyer-friendly

Sweeps the frontend for generic empty, loading, and error strings
and replaces them with action-oriented, lawyer-friendly copy that
explains what to do next. Adds a Try Again CTA to the global error
boundary so users aren't stuck on a dead page.

Highlights:
- Empty states (sidebar, file directory, modals, list pages, project
  detail) now name the next step (upload, create, ask, search by
  client/title) instead of just stating the absence.
- "Processing..." on a freshly uploaded doc → "Reading the document..."
  Upload buttons → "Uploading your document..."
- error.tsx and global-error.tsx now expose Next's reset() handler
  via a Try Again button; copy says Gary hit a problem and offers a
  return-home link.
- not-found.tsx CTA renamed Go home → Return Home.
- Login/signup error messages and progress labels rewritten
  ("Could not sign in. Check your email and password and try again.").
- Account/models alerts ("Failed to save X.") rewritten ("Could not
  save your X. Try again.").
- Tabular review streaming error: "An error occurred. Please try
  again." → "Something went wrong. Try asking again."

No backend changes, no behavior changes, no removed functionality.
Console/log/internal error details are unchanged.

https://claude.ai/code/session_019pRkhcGDRKQWHjzAnV5yCL