feat(agent+ui): fullstack agent module refactoring + UI/UX improvements

## Backend: agent module GRACE-Poly compliance
- Split app.py (749→~280 lines) into _tool_resolver, _confirmation, _persistence
- All 18 naked functions wrapped in #region/#endregion contracts
- Fixed @DEFGROUP→@defgroup typos; added @DATA_CONTRACT, @SIDE_EFFECT, CoT logs
- Conversation list API: added last_role, has_tool_calls, has_error, risk_level fields
- Message state detection: Russian/English error patterns (недоступен, unavailable)
- State field preserved in save_conversation messages
- HITL titles: descriptive tool names instead of generic "HITL resume"

## Backend: conversation title generation (two-layer)
- Layer 1: clean_title() — rule-based, strips file markers, pre-fetch blocks, JSON/CSV,
  URLs, code; truncates at 80 chars word boundary (25 unit tests, all edge cases)
- Layer 2: generate_llm_title() — async best-effort LLM titling via /v1/chat/completions
  with per-conversation lock, graceful degradation on failure

## Frontend: conversation list indicators (orthogonal system)
- Status dot (green/yellow/red/blue) per conversation state
- Icon column: tool activity, errors, waiting, completed
- Risk stripe (left border accent) + message count badge + relative time
- Fixed group labels: "Сегодня"/"Вчера" instead of "3 ч"/"5 ч"
- Hide "Окружение: —" when env is empty

## Frontend: guardrails card verification + fixes
- Confirmed all interaction modes: Enter/click confirm, Escape/click deny
- Auto-populate envId from environmentContextStore in DashboardDetailModel
- Better error message: missing_context_hint with recovery guidance

## Design system: semantic tokens
- Added category-* gradient tokens to tailwind.config.js
- Sidebar + Breadcrumbs use semantic tokens (10 categories)
- Raw Tailwind reduced from ~50 to 6 occurrences
- Added skip-to-content link in root layout (+layout.svelte)
- Added aria-label on DashboardDataGrid row checkboxes

## Protocol: INV_7 pragmatic exception
- Modules may exceed 400 lines when contract-dense (every function has #region)
- Recorded in semantics-core SKILL.md with rationale

Total: 5841+ contracts, 2993+ edges, backend 41/41, frontend 2501/2501

backend/src/agent/langgraph_setup.py

@@ -1,6 +1,6 @@
 # backend/src/agent/langgraph_setup.py
 # #region AgentChat.LangGraph.Setup [C:4] [TYPE Module] [SEMANTICS agent-chat,langgraph,agent]
-# @DEFGROUP AgentChat LangGraph agent setup: create_react_agent with PostgresSaver.
+# @defgroup AgentChat LangGraph agent setup: create_react_agent with PostgresSaver.
 # @PRE LLM provider configured. Priority: 1) llm_config param 2) env vars LLM_API_KEY/LLM_BASE_URL/LLM_MODEL.
 # @POST Compiled StateGraph ready for astream_events().
 # @SIDE_EFFECT Initializes checkpointer and message history tables on first call.