Phase 4: Domain swap (legal → finance) + Mike → Gordon rename

From the PR description

Summary

• Repositions the fork from legal AI to a finance AI assistant: rewrites system prompts, disclaimers, practice areas, and column presets for M&A diligence, PE, private credit, lev fin, IB, and equity/credit research.
• Completes the Mike → Gordon brand rename across types (MikeMessage/Workflow/Chat/...), components (MikeIcon), API client (mikeApi.ts), MIME types, localStorage key, R2 bucket default, README, and docs.
• Keeps all 12 built-in workflows untouched - finance workflow library is Phase 11. Old workflow practice values still display through the Practice = string flow + "Others" fallback.

What changed

Visible copy

• InitialView disclaimer now covers financial/investment/accounting/legal advice.
• Layout metadata (title, OG, Twitter), login/signup/support pages rebranded.
• Chat title generator prompt reframed for finance.

System prompts

• chatTools.ts SYSTEM_PROMPT rewritten with finance framing + citation-auditability emphasis.
• projectChat.ts project-context block now describes finance deal/mandate workstreams.
• tabular.ts extraction & review prompts (legal document analyst → finance document analyst).
• generate_docx tool description updated for finance artifacts (memo, IC paper, term sheet, diligence report, contract).

Practice areas - wholesale replacement: 19 legal practices → 17 finance verticals.

Column presets - additive, not replacement. Kept neutral legal contract presets (apply to finance contracts too) and added 12 finance presets (Revenue, EBITDA, Net Income, FCF, Leverage Ratio, Interest Coverage, Reporting Period, Maturity, Coupon/Rate, Currency, Capex, Margin).

Rename mechanics

• 9 type identifiers across 48 files.
• File renames: mike-icon.tsx → gordon-icon.tsx; mikeApi.ts → gordonApi.ts.
• MIME types application/mike-* → application/gordon-*, localStorage key, R2 bucket default in .env.example.
• FORK.md upstream URLs to willchen96/mike intentionally preserved.

Deferred per plan

• All 12 built-in workflows kept (Phase 11 will build the finance workflow library).
• LEGAL_NUMBERING_REF in shared/generateDocx.ts left as-is (it refers to Word's built-in legal numbering style - forces arabic numerals at all levels - not legal documents).

Full rationale in decisions.md.

Test plan

• tsc --noEmit clean on backend
• tsc --noEmit clean on frontend
• Backend Vitest: 57/57 passing
• Spot-check chat title generation produces finance-style titles
• Spot-check tabular review on a finance doc uses the new finance column presets
• Verify login/signup pages render with new Gordon branding
• Verify localStorage key change (mike.selectedModel → gordon.selectedModel) - users will re-select model on first load

Operational notes

• .env.example default R2_BUCKET_NAME changed mike → gordon. Existing deployments setting this var explicitly are unaffected; deployments relying on the default need to set the var or create a new bucket.
• localStorage key rename will silently reset users' last-selected model on their next visit. Acceptable one-time friction.

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