feat: add languages and file tabs

[mattenarle10]

Summary

• adds a lightweight i18next-based language selector with 9 complete locale files
• adds an open-file tab strip with dirty indicators, close confirmation, active-tab auto-scroll, and wheel scrolling for crowded tabs
• keeps code block wrapping from [feedback] Feature suggestions: i18n, tabs, and improved code block rendering #40 intact and completes the remaining i18n + tabs scope

Closes #40 when merged.

Release notes

• User-facing: language selector in the settings/theme menu, plus tabs for working across multiple markdown files.
• Download/app naming follow-up: [bug] App name #44 is separate from this feature PR. Current Tauri config uses productName/window title marka.md, while the Rust package is marka; before release, decide whether the installed app should display as marka.md or Marka MD, then keep macOS app bundle and release artifact names consistent.

Checks

• bun test
• bun run build
• git diff --check