fix(editor): replace broken image toolbar popovers with custom dialogs

[j4rviscmd]

Summary

Closes #40

• Replace BlockNote's built-in FileCaptionButton and FileRenameButton (Radix UI Popovers nested inside Floating UI popover — "Popover-in-Popover" issue causes toolbar dismissal) with custom @base-ui/react/dialog-based components rendered at the Editor level
• Replace built-in FileDownloadButton (window.open blocked in Tauri) with a Rust-backed download_file command that handles both local asset URLs and remote HTTP URLs
• Remove unused replaceFileButton and filePreviewButton from the toolbar
• Toolbar order: Rename → Edit Caption → Download → Delete → (alignment buttons)

Changes

File	Change
CaptionButton.tsx	New — Toolbar button that triggers caption dialog
CaptionDialog.tsx	New — Dialog for editing image captions (IME-safe)
RenameButton.tsx	New — Toolbar button that triggers rename dialog
RenameDialog.tsx	New — Dialog for renaming image blocks (IME-safe)
DownloadButton.tsx	New — Downloads via Rust download_file command
Editor.tsx	Updated — Replace pass-through buttons with custom components
file_io.rs	Updated — Add download_file Tauri command
lib.rs	Updated — Register download_file in invoke handler

Test plan

• Click image → bubble menu appears
• Click "Edit caption" → dialog opens, type + Enter saves
• Click "Rename" → dialog opens, type + Enter saves
• Click "Download" → save dialog with correct extension, file saved
• IME composition Enter does not trigger save
• Toolbar order: Rename → Caption → Download → Delete

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