Fix ObsidianReviewBot lint issues for plugin submission

- Remove async from focusFolder function (no await expressions)
- Remove unnecessary type assertions (folder-ops.ts, main.ts)
- Replace TFile casts with instanceof checks for type safety
- Add ObsidianAppWithPlugins interface to replace `any` types
- Remove `this` aliasing, use bound methods instead
- Fix Promise in void context by wrapping async callbacks
- Apply sentence case to UI text ("Generate Default" → "Generate default")

Author: ZainRizvi