feat: add guillemet auto-formatting (<< and >>) to the document editor

[kyo5uke]

Feature Preview

This adds two unambiguous auto-typography conversions to the document editor, building on the existing -- → —, -> → → and => → ⇒ shortcuts:

• << → «
• >> → »

Both reuse the existing _handleDoubleCharacterReplacement helper (so the conversion is undoable) and are registered in buildCharacterShortcutEvents. The >> handler keys off a preceding >, so it does not conflict with the existing => / -> shortcuts (which look for = / -) or the > + space block-quote toggle.

This is a small first step toward #3322. The other requests in that issue (three-character conversions such as (c) → ©, the ambiguous quote conversions, and an on/off "Advanced settings" toggle) are intentionally left out of this PR and can be handled separately.

Partially addresses #3322

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PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.

Summary by Sourcery

Add new auto-typography shortcuts in the document editor to convert double angle brackets into guillemets and wire them into the existing character shortcut system.

New Features:

• Convert << into the left guillemet character « via a character shortcut event in the document editor.
• Convert >> into the right guillemet character » via a character shortcut event in the document editor.

Tests:

• Add unit tests verifying that << and >> are converted to « and » respectively and that the operations are undoable.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

Reviewer's Guide

Adds new document editor character shortcuts that auto-convert double angle brackets (<<, >>) into guillemets («, »), wired into the existing double-character replacement infrastructure and covered by unit tests including undo behavior.

Sequence diagram for new guillemet auto-formatting shortcuts

sequenceDiagram
  actor User
  participant Editor as DocumentEditor
  participant Shortcuts as CharacterShortcutEvent
  participant Handler as _handleDoubleCharacterReplacement

  User->>Editor: type "<"
  Editor->>Shortcuts: customFormatDoubleAngleLeftGuillemet
  Shortcuts->>Handler: _handleDoubleCharacterReplacement(editorState, character: _leftAngle, replacement: _leftGuillemet)
  Handler-->>Editor: replace "<<" with "«" (undoable)

  User->>Editor: type ">"
  Editor->>Shortcuts: customFormatDoubleAngleRightGuillemet
  Shortcuts->>Handler: _handleDoubleCharacterReplacement(editorState, character: _greater, replacement: _rightGuillemet)
  Handler-->>Editor: replace ">>" with "»" (undoable)

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File-Level Changes

Change	Details	Files
Introduce guillemet double-angle auto-formatting using the existing double-character replacement helper.	Define constants for left/right angle brackets and their corresponding guillemet characters. Create customFormatDoubleAngleLeftGuillemet and customFormatDoubleAngleRightGuillemet CharacterShortcutEvents that call _handleDoubleCharacterReplacement with the appropriate trigger character and replacement. Register the new guillemet shortcuts in the buildCharacterShortcutEvents list so they are active across desktop, mobile, and web.	frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/base/format_arrow_character.dart frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/shortcuts/character_shortcuts.dart
Add unit tests to validate guillemet auto-formatting and undo behavior.	Initialize a blank document with a single paragraph node containing a single angle bracket and place the cursor after it. Execute the appropriate custom guillemet format command and assert that the document text becomes the guillemet character. Execute undoCommand and assert that the document reverts to the original double angle bracket text, then dispose the editor state.	frontend/appflowy_flutter/test/unit_test/document/shortcuts/format_shortcut_test.dart

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