refactor(editor): migrate to ProseMirror Mark-based ghost text system

- Replace overlay-based GhostTextOverlay.vue with ProseMirror Mark system
- Add AI toggle button with enable/disable functionality
- Implement new copilotPlugin.ts using copilotGhostMark for inline suggestions
- Fix cursor position offset in prompt.py by moving first suffix char to prefix
- Improve API error handling with abort signal support and debug logging
- Update model configuration from gpt-oss:120b to gpt-oss:20b
- Add button tooltips and improve editor styling
- Remove deprecated inlineSuggestionPlugin.ts
- Update README with new architecture diagram and feature documentation

backend/prompt.py

@@ -8,6 +8,12 @@ def build_prompt(prefix: str, suffix: str) -> str:
     """
     MAX_CONTEXT_LINES = 30
     
+    # 修正：把suffix的第一个字符移到prefix末尾（解决光标位置偏差）
+    if suffix:
+        first_char = suffix[0]
+        prefix = prefix + first_char
+        suffix = suffix[1:]
+    
     prefix_lines = prefix.split('\n')
     suffix_lines = suffix.split('\n') if suffix else []
     
@@ -25,6 +31,7 @@ RULES:
 - Match existing style, tone, and terminology
 - Maintain logical flow
 - Write only the continuation, nothing else
+- IMPORTANT: Start continuation directly from ⟨CURSOR⟩ position
 
 TEXT:
 {recent_prefix}⟨CURSOR⟩{recent_suffix}