Resolve all mypy typing errors and enable full-project type checking

[dasarne]

Background

The CI pipeline currently limits mypy checks to the stable core modules because the Qt UI layer still contains a large number of typing issues.

Current status:

• Tests pass on Python 3.10–3.12
• Qt headless CI works correctly
• Core modules are type-checked
• Full-project mypy currently reports ~116 errors

Affected areas include:

• src/ui/editor_panel.py
• src/ui/canvas_panel.py
• src/ui/main_window.py
• src/ui/widgets.py
• src/ui/comment_panel.py
• src/ui/settings_dialog.py
• src/ui/warnings_dialog.py

Main Error Categories

Qt override signatures

Examples:

• paintEvent(self, event)
• resizeEvent(self, event)
• mousePressEvent(self, event)

PyQt6 stubs require signatures like:

• QPaintEvent | None
• QResizeEvent | None
• QMouseEvent | None

Optional Qt return values

Examples:

• menuBar()
• statusBar()
• document()
• clipboard()
• horizontalHeader()

These are currently treated as always available but typed as optional in PyQt6 stubs.

Painter overload typing

Examples:

• QPainter.drawLines(list[QLineF])

PyQt6 expects array-compatible overloads.

Goals

• Eliminate all current mypy errors
• Re-enable full-project mypy checking in CI
• Improve maintainability and IDE support
• Prepare the UI layer for stricter future typing

Requirements

• No functional regressions
• No UX regressions
• Preserve editor responsiveness
• Preserve rendering performance
• Avoid broad type: ignore usage
• Prefer explicit typing and safe Optional handling
• Keep changes incremental and reviewable

Suggested Migration Order

1. Fix override signatures
2. Fix Optional Qt object handling
3. Fix painter/container overload typing
4. Add missing annotations
5. Re-enable full mypy src/ in CI

Acceptance Criteria

• mypy src/ passes without errors
• CI fully green on all Python versions
• No runtime regressions
• No unnecessary suppression comments
• Typing quality improved across UI modules