Align repo config with global development standards

- Add missing tsconfig strictness flags (noUncheckedIndexedAccess,
  exactOptionalPropertyTypes, noImplicitOverride,
  noPropertyAccessFromIndexSignature) and fix all resulting type errors
- Replace ESLint/Prettier with oxlint 1.48.0 and oxfmt 0.33.0
- Pin all frontend and backend dependencies to exact versions
- Pin GitHub Actions to SHA hashes with persist-credentials: false
- Fix CI Python version mismatch (3.12 -> 3.14) and ruff target-version
- Add vitest 4.0.18 with jsdom environment for frontend testing
- Add ty 0.0.17 for Python type checking (non-blocking in CI)
- Add actionlint and zizmor CI job for workflow linting and security audit
- Add Dependabot config for npm, pip, and github-actions
- Update CLAUDE.md and pre-commit hooks to reflect new tooling
- Ignore Claude Code sandbox artifacts in gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -16,7 +16,7 @@ This project uses [pre-commit](https://pre-commit.com/) to run linting and forma
 
 **Hooks configured:**
 - **Backend:** `ruff check --fix` and `ruff format` on Python files under `backend/`
-- **Frontend:** `eslint`, `prettier --check`, and `tsc -b` on files under `frontend/`
+- **Frontend:** `oxlint`, `oxfmt --check`, and `tsc -b` on files under `frontend/`
 
 Frontend hooks require `npm ci` in `frontend/` first (they use `npx` to run from local `node_modules`).