refactor: unified EditorPanel for notes and files, SaveFileText backend

- Create EditorPanel.svelte: shared editor component for notes + files
  - Markdown: edit/preview/split modes via MarkdownEditor + MarkdownPreview
  - Plain text: monospace textarea
  - Header: title, mode switcher, external open button
  - Footer: save + close buttons, dirty indicator
- Rewrite NoteEditorPanel.svelte as thin wrapper over EditorPanel
- FilesTab: browser/editor modes (editor replaces file list, not appended)
- Add SaveFileText Wails binding: writes text files atomically (tmp+rename)
- Add WriteText to files.Service with SHA256 update
- Remove separate TextFileEditor.svelte (replaced by EditorPanel)
- Note context: SaveNote API, noteId/title props
- File context: SaveFileText API, fileId prop
- Linked .md (CheckFileAction=note) → note editor tab
- Unlinked .md / text files → EditorPanel in FilesTab
- Images/PDF/binary → FilePreviewModal only

Co-Authored-By: OWL (Hermes Agent) <hermes@nousresearch.com>

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