refactor(settings): replace Obsidian 1.13 compatibility hotfix

[ClaudiaFang]

Context

Issue #98 was fixed by PR #99 with a compatibility adapter that returns an empty getSettingDefinitions() result, keeping the existing imperative display() lifecycle active on Obsidian 1.13+.

This is intentionally a hotfix. The settings implementation should be cleaned up so the compatibility behavior is explicit, testable, and no longer depends on the temporary module split.

Improvement plan

• Remove the temporary settings.ts / settings-implementation.ts adapter split.
• Decide on one supported lifecycle:
  • keep the imperative display() implementation and remove the declarative compatibility method entirely, or
  • fully migrate the settings UI to Obsidian's declarative settings API.
• Make refresh behavior explicit for provider and language changes instead of relying on inherited update() behavior.
• Add regression coverage for:
  • initial rendering on Obsidian 1.13+;
  • refresh after provider changes;
  • refresh after language changes;
  • reopening the settings tab;
  • compatibility with the oldest supported Obsidian version.
• Improve the Obsidian Setting test mock so DOM assertions can distinguish framework markup from plugin-owned markup.
• Document the supported Obsidian version range and settings API strategy.

Acceptance criteria

• The settings page renders and refreshes correctly on all supported Obsidian versions.
• No temporary compatibility adapter or duplicated settings module remains.
• Lifecycle and refresh behavior are covered by regression tests.
• pnpm test, pnpm lint, and pnpm build pass.

Follow-up to #98 and PR #99.