Add snippet/command library with CRUD and send-to-session

Full command snippet library for saving and sending reusable commands:

Backend:
- SQLite migration 003 creates snippets table
- Snippet model with name, command, description, folder, tags
- Store CRUD methods and API endpoints

Frontend:
- SnippetLibrary class with searchable list + detail/editor split view
- Create, edit, delete snippets in modal dialog
- 'Send to Session' button writes command to active terminal
- 'Send Snippet' option in terminal right-click context menu
- Ctrl+Shift+S keyboard shortcut to open library

internal/storage/migrations/003_snippets.sql

@@ -0,0 +1,15 @@
+-- Migration 003: Add snippets table for the command snippet library.
+-- Stores reusable commands that can be sent to active sessions.
+
+CREATE TABLE IF NOT EXISTS snippets (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    command TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    folder TEXT NOT NULL DEFAULT '',
+    tags TEXT NOT NULL DEFAULT '[]',
+    created_at TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+INSERT INTO schema_version (version, applied_at) VALUES (3, datetime('now'));