Agent loop state machine refactor, unified LLM interface, and UI improvements

- Rewrite agent loop as Planning→Executing(N)→Completed state machine with
  per-step context isolation to prevent token explosion
- Split tools and prompts by phase (planning vs execution)
- Add advance_step/save_memo tools for step transitions and cross-step memory
- Unify LLM interface: remove duplicate types, single chat_with_tools path
- Add UTF-8 safe truncation (truncate_str) to prevent panics on Chinese text
- Extract CreateForm component, add auto-scroll to execution log
- Add report generation with app access URL, non-blocking title generation
- Add timer system, file serving, app proxy, exec module
- Update Dockerfile with uv, deployment config

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

Cargo.toml

@@ -24,3 +24,5 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1", features = ["v4"] }
 anyhow = "1"
+mime_guess = "2"
+nix = { version = "0.29", features = ["signal"] }