transcription: add dictionary and AI enhancement

[BumpyClock]

Important

Stacked on #21. Merge #21 first, then retarget this PR to main.

Summary

• add deterministic custom dictionary replacements, including whole-term C++, C#, and .NET handling, and Whisper vocabulary hints
• add optional OpenAI-compatible enhancement through OpenAI, Ollama, or LM Studio
• keep API keys in environment variables and redact provider bodies/transcripts from errors
• reconcile enhanced final text with live preview and preserve deterministic fallback
• add ASR real-time-factor telemetry and coalesce model warm-up work
• require the installed Whisper sidecar manifest to match the app protocol and expose an update action for stale installs
• expose settings UI and document provider trust boundaries and live-preview limits
• ignore .pi-subagents/ and commit the approved Claude plugin settings

Security and reliability

• OpenAI keys are read from the configured env var only
• custom OpenAI base URLs receive that bearer key and must be trusted
• every non-loopback provider endpoint requires HTTPS; loopback HTTP remains supported
• credentials, queries, and fragments in provider base URLs are rejected
• automatic redirects and system proxy discovery are disabled for AI enhancement requests
• endpoint selection compares parsed hosts, not substrings
• response bodies are streamed with a 1 MiB limit and transcript contents are excluded from parse/status errors
• incomplete Chat Completions and Responses API payloads are rejected
• provider-dependent model defaults survive persisted-settings deserialization
• malformed persisted dictionary entries are dropped without resetting unrelated settings
• transcription finalization blocks overlapping captures and cleans up trusted previews on model removal

Testing

• cargo fmt --all -- --check
• cargo check --workspace --all-targets --locked
• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --locked -- --deny warnings

All checks pass on Rust 1.95, including the complete workspace test suite.

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