feat: re-center the Obsidian plugin around the Python backend runtime

[maehwasoo]

What

• Re-center the plugin runtime UI around the Python backend with a primary backend status bar/modal and Python-first startup order
• Reword settings and commands so MCP is treated as an optional transition service
• Update plugin/runtime docs to describe the Python-centered local runtime model

Why

• Issue feat: re-center the Obsidian plugin around the Python backend runtime #180 tracks the plugin transition from split runtime ownership to a Python-centered local backend model

• The plugin already launches the Python backend, but startup order and status/troubleshooting UX still emphasized transition-first assumptions

• Fixes feat: re-center the Obsidian plugin around the Python backend runtime #180

How

• Add a Python backend status bar/modal, a Python backend status command, and a small startup helper that boots Python before MCP when both are enabled
• Keep MCP and indexer explicit as transition components in settings copy and README/overview docs
• Add focused tests for Python-first startup order and backend status bar rendering, then verify with pnpm check:ci