Add multi-backend support to example app

[linroid]

Summary

• BackendManager manages embedded and remote KDown instances with lifecycle management, switching, and connection state tracking
• Embedded KDown instance is created once and reused across backend switches and local server startup
• Local HTTP server controls merged into the Embedded entry in the backend selector UI (no separate "Local Server" backend type)
• Android and Desktop both support starting a local HTTP server exposing the embedded instance
• WebApp rewritten to use the shared Compose UI (App(backendManager)) in remote-only mode
• SqliteTaskStore used for persistence on Android, iOS, and Desktop
• Removed redundant BackendType enum (type derived from BackendConfig sealed class)
• Removed redundant BackendConnectionState (reuses ConnectionState from the remote module directly)

Key architecture

• BackendFactory uses lambda injection for platform-specific capabilities (embedded creation, local server, SQLite driver)
• BackendManager stores a single embedded KDownApi instance, reused for both direct use and local server exposure
• DisconnectedApi null-object pattern avoids nullable activeApi in remote-only mode (wasmJs)
• BackendEntry.isEmbedded derived property replaces the deleted BackendType enum

Test plan

• All platforms build: Android, Desktop (JVM), iOS, wasmJs, webApp
• JVM unit tests pass (BackendManagerTest, FakeKDownApi, FakeBackendFactory)
• Manual: verify embedded backend works on Android/Desktop
• Manual: verify local server start/stop on Android/Desktop
• Manual: verify remote backend connection from webApp
• Manual: verify backend switching in the selector sheet

🤖 Generated with Claude Code

[github-actions]

Test Results

756 tests  +84   756 ✅ +84   11s ⏱️ ±0s
 63 suites + 3     0 💤 ± 0 
 63 files   + 3     0 ❌ ± 0 

Results for commit 7063722. ± Comparison against base commit 5b3d423.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.