fix(server): declare UTF-8 for SPA HTML responses

[mzkarami]

Thinking Path

• Paperclip orchestrates AI agents for zero-human companies.
• Public issue Incorrect character encoding for Danish characters when using Paperclip AI in Danish  paperclipai/paperclip#7310 reports Danish and accented characters rendering with incorrect encoding.
• The current draft makes the SPA HTML fallback responses explicitly declare UTF-8 and adds a small Danish-text regression test.
• Local and fork CI validation pass for the code change itself.
• A fresh upstream duplicate check found open PRs that already touch the same encoding area, including fix(encoding): use StringDecoder to prevent UTF-8 corruption in UI paperclipai/paperclip#3041, which changes the same SPA fallback HTML response headers in server/src/app.ts.
• Because fix(encoding): use StringDecoder to prevent UTF-8 corruption in UI paperclipai/paperclip#3041 overlaps the same behavior/path, this fork draft should not be submitted upstream as-is unless a maintainer asks for a narrower alternative or for the regression-test coverage specifically.

What Changed

• Set the static SPA fallback response type to text/html; charset=utf-8.
• Set the Vite dev HTML response type to text/html; charset=utf-8.
• Added a regression test that verifies fallback HTML declares UTF-8 and preserves Danish text (Ærøskøbing blåbærgrød).

Verification

• git diff --check
• pnpm exec vitest run server/src/__tests__/static-index-html.test.ts
• pnpm --filter @paperclipai/server typecheck
• Fork PR CI PR workflow succeeded for head 3ce04701b1b1ad068fc1a1d08cd31c79efdc0a6d

Risks

• Low technical risk. This only makes the HTML response charset explicit for browser clients; no database, API contract, or routing behavior changes.
• Submission risk: high duplicate/overlap risk because open upstream PR fix(encoding): use StringDecoder to prevent UTF-8 corruption in UI paperclipai/paperclip#3041 already changes the same SPA fallback HTML header lines, and Enforce UTF-8 encoding for DB connections, HTTP responses (issue #2928) paperclipai/paperclip#3557/fix(windows): UTF-8 + cross-platform encoding robustness paperclipai/paperclip#4427 overlap the broader UTF-8 response/encoding theme.

For core feature work, check ROADMAP.md first and discuss it in #dev before opening the PR. Feature PRs that overlap with planned core work may need to be redirected — check the roadmap first. See CONTRIBUTING.md.

Model Used

• OpenAI Codex via Hermes Agent, model gpt-5.5, with repository file access, shell command execution, and GitHub CLI tool use.

Checklist

• I have included a thinking path that traces from project context to this change
• I have specified the model used (with version and capability details)
• I have checked ROADMAP.md and confirmed this PR does not duplicate planned core work
• I have run tests locally and they pass
• I have added or updated tests where applicable
• If this change affects the UI, I have included before/after screenshots
• I have updated relevant documentation to reflect my changes
• I have considered and documented any risks above
• I will address all Greptile and reviewer comments before requesting merge

Public discovery / duplicate checks

• Public issue considered: Incorrect character encoding for Danish characters when using Paperclip AI in Danish  paperclipai/paperclip#7310 (incorrect Danish/accented character encoding).
• Open upstream PRs/issues searched: charset utf-8 html Danish encoding, UTF-8 OR utf8 OR charset OR Danish OR encoding.
• Related open upstream PRs found:
  • fix(encoding): use StringDecoder to prevent UTF-8 corruption in UI paperclipai/paperclip#3041 — changes the same server/src/app.ts SPA fallback HTML content-type headers.
  • Enforce UTF-8 encoding for DB connections, HTTP responses (issue #2928) paperclipai/paperclip#3557 — broader UTF-8 response/database/child-process handling.
  • fix(windows): UTF-8 + cross-platform encoding robustness paperclipai/paperclip#4427 — broader Windows UTF-8/cross-platform encoding handling, including app response headers.
• Result: do not submit this draft upstream as-is; it is useful as a reviewed narrow patch/test reference, but likely duplicates existing open work.

Human gate

This is a fork draft prepared for human review only. Upstream submission requires explicit human approval in a separate interactive session. Current recommendation: do not submit upstream as-is because of duplicate/overlap risk.

[mzkarami]

Closing this fork draft after review: the change overlaps existing upstream encoding PRs (paperclipai#3041/paperclipai#3557/paperclipai#4427), so it is not worth submitting upstream as-is.