Add schema-aware SQL auto-complete to the SQL editor

New file app/ui/sql_completer.py:
- SqlCompleter (QObject) owns a _CompletionPopup (QListWidget) that
  floats above the editor as a ToolTip-style frameless window styled
  to match the Catppuccin Mocha theme.
- SQL keyword pool sourced directly from syntax_highlighter._KEYWORDS,
  _TYPES, _FUNCTIONS so the two features stay in sync.
- Schema data (tables, views, columns) loaded asynchronously via
  SchemaWorker; a generation counter discards stale results when the
  database changes rapidly.
- Column loading is lazy: fires only when the user types "table."
  (dot-qualified prefix), avoiding upfront cost for wide schemas.
- _extract_prefix() uses a regex to detect both bare words and
  "table.col" dot notation; guards against completing inside string
  literals by counting unescaped quotes to the left of the cursor.
- _accept_completion() replaces only the typed prefix, preserving any
  qualifier already in the document (e.g. "orders.cu" → "orders.customer_id").

Changes to app/ui/sql_editor.py:
- CodeEditor.set_completer() attaches the completer and stores it.
- keyPressEvent priority order:
    1. Popup visible → Esc hides, Enter/Tab accepts, Up/Down navigates.
    2. Ctrl+Space → force-show completions (1-char minimum).
    3. Tab (no popup) → existing 4-space insert behaviour preserved.
    4. Ctrl+/ → existing comment-toggle preserved.
    5. All other keys → super() then auto-trigger (2-char minimum).
- focusOutEvent hides the popup when the editor loses focus.
- EditorTab._build_ui() creates SqlCompleter, attaches it, and calls
  set_context() if a driver is already present.
- EditorTab.set_context() updates driver/database and reloads schema.
- _maybe_invalidate_schema() re-fetches schema after CREATE/DROP/ALTER
  so newly created tables appear in completions immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/ui/sql_editor.py

@@ -15,6 +15,7 @@ from PyQt6.QtGui import (
 
 from app.ui.syntax_highlighter import SQLHighlighter
 from app.ui.results_panel import ResultsPanel
+from app.ui.sql_completer import SqlCompleter
 from app.utils.worker import QueryWorker
 
 
@@ -33,11 +34,12 @@ class LineNumberArea(QWidget):
 
 
 class CodeEditor(QPlainTextEdit):
-    """QPlainTextEdit with line numbers, current-line highlight, and tab→spaces."""
+    """QPlainTextEdit with line numbers, current-line highlight, tab→spaces, and auto-complete."""
 
     def __init__(self, parent=None):
         super().__init__(parent)
         self._line_area = LineNumberArea(self)
+        self._completer: "SqlCompleter | None" = None
 
         # Font
         font = QFont("Consolas", 13)
@@ -109,19 +111,61 @@ class CodeEditor(QPlainTextEdit):
             bottom = top + round(self.blockBoundingRect(block).height())
             number += 1
 
+    def set_completer(self, completer: "SqlCompleter") -> None:
+        self._completer = completer
+
+    def _popup_visible(self) -> bool:
+        return (self._completer is not None
+                and self._completer._popup.isVisible())
+
+    def focusOutEvent(self, event):
+        super().focusOutEvent(event)
+        if self._completer:
+            self._completer._popup.hide_popup()
+
     def keyPressEvent(self, event):
-        # Tab → 4 spaces
-        if event.key() == Qt.Key.Key_Tab:
-            cursor = self.textCursor()
-            cursor.insertText("    ")
+        # ── Popup navigation (highest priority) ──────────────────────────────
+        if self._popup_visible():
+            key = event.key()
+            if key == Qt.Key.Key_Escape:
+                self._completer._popup.hide_popup()
+                return
+            if key in (Qt.Key.Key_Return, Qt.Key.Key_Enter, Qt.Key.Key_Tab):
+                self._completer._accept_completion(
+                    self._completer._popup.current_text()
+                )
+                return
+            if key == Qt.Key.Key_Up:
+                self._completer._popup.move_selection(-1)
+                return
+            if key == Qt.Key.Key_Down:
+                self._completer._popup.move_selection(1)
+                return
+
+        # ── Ctrl+Space → force-show completions ──────────────────────────────
+        if (event.modifiers() == Qt.KeyboardModifier.ControlModifier
+                and event.key() == Qt.Key.Key_Space):
+            if self._completer:
+                self._completer.trigger_completion(force=True)
             return
-        # Ctrl+/ → toggle comment
-        if event.modifiers() == Qt.KeyboardModifier.ControlModifier \
-                and event.key() == Qt.Key.Key_Slash:
+
+        # ── Tab → 4 spaces (only when popup is closed) ───────────────────────
+        if event.key() == Qt.Key.Key_Tab:
+            self.textCursor().insertText("    ")
+            return
+
+        # ── Ctrl+/ → toggle comment ───────────────────────────────────────────
+        if (event.modifiers() == Qt.KeyboardModifier.ControlModifier
+                and event.key() == Qt.Key.Key_Slash):
             self._toggle_comment()
             return
+
         super().keyPressEvent(event)
 
+        # ── Auto-trigger completions after every regular keystroke ────────────
+        if self._completer:
+            self._completer.trigger_completion(force=False)
+
     def _toggle_comment(self):
         cursor = self.textCursor()
         start  = cursor.selectionStart()
@@ -201,6 +245,10 @@ class EditorTab(QWidget):
 
         self._editor = CodeEditor()
         SQLHighlighter(self._editor.document())
+        self._completer = SqlCompleter(self._editor, parent=self)
+        self._editor.set_completer(self._completer)
+        if self._driver:
+            self._completer.set_context(self._driver, self._database)
 
         self._results = ResultsPanel()
         self._results.status_message.connect(self.status_message)
@@ -264,20 +312,33 @@ class EditorTab(QWidget):
     def _export(self):
         self._results.export_dialog()
 
+    def set_context(self, driver, database: str) -> None:
+        """Update driver/database and refresh schema completions."""
+        self._driver   = driver
+        self._database = database
+        self._db_label.setText(f"DB: {database}" if database else "")
+        self._completer.set_context(driver, database)
+
     def _on_result(self, cols, rows, cnt, elapsed):
         self._results.show_data(cols, rows, cnt, elapsed)
+        self._maybe_invalidate_schema(self._editor.toPlainText())
 
     def _on_script_done(self, results: list):
-        # Show the last SELECT result; messages for DML
         for cols, rows, cnt, msg in results:
             if cols:
                 self._results.show_data(cols, rows, cnt, 0)
             else:
                 self._results.show_message(msg)
+        self._maybe_invalidate_schema(self._editor.toPlainText())
 
     def _on_error(self, msg: str):
         self._results.show_error(msg)
 
+    def _maybe_invalidate_schema(self, sql: str) -> None:
+        if any(sql.lstrip().lower().startswith(kw)
+               for kw in ("create", "drop", "alter", "rename")):
+            self._completer.invalidate()
+
     # ── Public ─────────────────────────────────────────────────────────────────
 
     def set_sql(self, sql: str):