feat: add Obsidian Integration

[ddxkalin]

Summary

• Move Obsidian integration into a new Extensions page above Settings.
• Add install/manage/uninstall flow for the Obsidian extension.
• Save normal transcriptions to Obsidian when the extension is installed.
• Always write Obsidian notes under <Vault>/Yap/.
• Add configurable Obsidian note names with {{date}} support and safe filename sanitization.
• Add official Obsidian logo asset and updated extension UI styling.
• Improve onboarding hotkey test recovery when transcription setup fails.
• Add diagnostics logging for setup/transcription issues.

Behavior

• Default Obsidian note path is Yap/Transcriptions YYYY-MM-DD.md.
• Empty note name resolves to Transcriptions {{date}}.
• Obsidian write failures are non-fatal: transcription history, audio saving, stats, and copy/autopaste still complete.

Testing

• go test ./...
• npm run build

Notes

• Wails generated runtime files and go.mod/go.sum changed after using a newer Wails CLI. The app does not directly use the newly generated notification runtime APIs.