TP-25 — Add user theme folder docs and open-folder affordance

[ZhuchkaTriplesix]

Epic: D
Depends on: TP-11, TP-21

Source: docs/theme-parser-github-issues.md

Goal

Make filesystem themes discoverable.

Files

• docs/theme-custom-json.md
• docs/theme-import.md
• lib/features/settings/preferences_appearance_section.dart

Implementation

Docs:

• show actual app support path behavior;
• mention accepted extensions;
• mention refresh/restart.

UI:

• show hint text:
  • "Themes are loaded from app support themes folder."
• optional button:
  • Open themes folder
  • can be follow-up if cross-platform opening helper does not exist.

Acceptance Criteria

• User can understand where to put downloaded themes.
• UI does not promise watcher/live reload if not implemented.

Tests

Docs only unless adding button.

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