fix(core,mcp,security)!: resolve critical P0/P1 issues

BREAKING CHANGES:
- owlen-core no longer depends on ratatui/crossterm
- RemoteMcpClient constructors are now async
- MCP path validation is stricter (security hardening)

This commit resolves three critical issues identified in project analysis:

## P0-1: Extract TUI dependencies from owlen-core

Create owlen-ui-common crate to hold UI-agnostic color and theme
abstractions, removing architectural boundary violation.

Changes:
- Create new owlen-ui-common crate with abstract Color enum
- Move theme.rs from owlen-core to owlen-ui-common
- Define Color with Rgb and Named variants (no ratatui dependency)
- Create color conversion layer in owlen-tui (color_convert.rs)
- Update 35+ color usages with conversion wrappers
- Remove ratatui/crossterm from owlen-core dependencies

Benefits:
- owlen-core usable in headless/CLI contexts
- Enables future GUI frontends
- Reduces binary size for core library consumers

## P0-2: Fix blocking WebSocket connections

Convert RemoteMcpClient constructors to async, eliminating runtime
blocking that froze TUI for 30+ seconds on slow connections.

Changes:
- Make new_with_runtime(), new_with_config(), new() async
- Remove block_in_place wrappers for I/O operations
- Add 30-second connection timeout with tokio::time::timeout
- Update 15+ call sites across 10 files to await constructors
- Convert 4 test functions to #[tokio::test]

Benefits:
- TUI remains responsive during WebSocket connections
- Proper async I/O follows Rust best practices
- No more indefinite hangs

## P1-1: Secure path traversal vulnerabilities

Implement comprehensive path validation with 7 defense layers to
prevent file access outside workspace boundaries.

Changes:
- Create validate_safe_path() with multi-layer security:
  * URL decoding (prevents %2E%2E bypasses)
  * Absolute path rejection
  * Null byte protection
  * Windows-specific checks (UNC/device paths)
  * Lexical path cleaning (removes .. components)
  * Symlink resolution via canonicalization
  * Boundary verification with starts_with check
- Update 4 MCP resource functions (get/list/write/delete)
- Add 11 comprehensive security tests

Benefits:
- Blocks URL-encoded, absolute, UNC path attacks
- Prevents null byte injection
- Stops symlink escape attempts
- Cross-platform security (Windows/Linux/macOS)

## Test Results

- owlen-core: 109/109 tests pass (100%)
- owlen-tui: 52/53 tests pass (98%, 1 pre-existing failure)
- owlen-providers: 2/2 tests pass (100%)
- Build: cargo build --all succeeds

## Verification

- ✓ cargo tree -p owlen-core shows no TUI dependencies
- ✓ No block_in_place calls remain in MCP I/O code
- ✓ All 11 security tests pass

Fixes: #P0-1, #P0-2, #P1-1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Cargo.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "crates/owlen-core",
+    "crates/owlen-ui-common",
     "crates/owlen-tui",
     "crates/owlen-cli",
     "crates/owlen-providers",