feat: add separate Code Mode for fenced code blocks

[TyceHerrman]

Summary

Add an optional Code Mode that keeps prose in the existing Notes index and
embeds fenced code blocks in a separate Code index with its own model. Results
remain mode-specific; scores from different embedding spaces are never mixed.
Code Mode is off by default, so existing Notes behavior is unchanged.

• Extract backtick and tilde fences with language and line metadata.
• Support independent Code embedding models through local Hugging
  Face/Transformers models with ONNX weights, Ollama, OpenAI-compatible APIs,
  and Google Gemini. Recommended built-ins are all-MiniLM-L6-v2 and
  paraphrase-multilingual-MiniLM-L12-v2; no code-specialized model is bundled.
• Give Code its own namespaced persistence, status, queues, fingerprints, and
  rebuild lifecycle.
• Add Notes/Code switching to the sidebar, bottom view, and Semantic Search.
• Make model and index changes transactional, with rollback and lifecycle gates
  that prevent stale work from publishing results.
• Document persistence, resource use, mobile constraints, and remote-provider
  privacy implications.

No new runtime dependencies, IndexedDB schema-version bump, release-version
bump, or changes to existing Notes persistence names.

Closes #52.

Test plan

• npm run test — 36 files, 240 tests
• npm run lint
• npm run build
• git diff --check

Automated coverage includes extraction and chunking, Notes/Code partitioning,
persistence and migration, mode-specific routing and caches, transactional
rollback, stale-result suppression, teardown, and unload behavior.

Manual Obsidian verification

Verified with disposable vaults on Obsidian 1.12.7 and 1.13.1. Build artifacts
were copied manually because install-local delegates to a gitignored,
developer-local helper that is not shipped with the repository.

• Confirm disabled mode preserves whole-note Notes indexing and exposes no
  Notes/Code selectors.
• Enable Code Mode and verify separate prose/code results in the sidebar,
  bottom view, and Semantic Search persist after restart.
• Verify fence removal, rename, deletion, and folder exclusion update both
  indexes without stale results.
• Rebuild with a different Code model and verify invalid-model rollback
  restores the previous usable model and index.
• Disable and remove the Code index; confirm status becomes Disabled,
  selectors disappear, and Notes search again retrieves fenced-code content.

See docs/code-mode-spec.md for the detailed behavior contract.