perf: eliminate per-frame allocations across all paint-hot paths and event dispatchers

[gmmcosta15]

Description

Select the type:

• Feature
• Bug fix
• Code refactor
• Documentation

Eliminates repeated per-frame CPU work in the application's paint-hot paths and event dispatchers, reducing idle CPU from ~73% GIL to ~2% GIL (measured via py-spy on Raspberry Pi 5 aarch64).

• list_model.py - QPainterPath cache keyed by (w, h) + painter.translate(); pre-computed QColor objects; icon scaled() cache keyed by QSize; QFont/QFontMetrics cache by point size; _get_tinted() LRU for tinted QPixmap composites; removed LosslessImageRendering hint; handlePath.clear() + addEllipse() path reuse
• display_button.py - round-rect QPainterPath cache by (w, h); _get_font(pt) helper caches QFont by point size; icon + secondary icon scaled() cache by QSize; QColor pre-computation for hover gradient and secondary text; removed LosslessImageRendering hint
• toggleAnimatedButton.py - icon scaled() cache keyed by QSize; handlePath.clear() reuse instead of fresh QPainterPath each frame
• blocks_button.py - background QPainterPath and icon scaled() cache keyed by QSize; removed duplicate CE_PushButtonLabel draw call and per-paint setStyle() call
• mainWindow.py - 5 custom event types pre-computed as class-level constants; single etype = event.type() call per event() dispatch instead of one per branch
• screensaver.py - _TOUCH_INTS: frozenset[int] stores raw .value ints; event.type().value in _TOUCH_INTS bypasses per-call SIP enum construction
• loadWidget.py - show() replaced with setVisible() override; 16ms animation timer is paused when the widget is hidden and resumed when shown
• files.py - installEventFilter narrowed to self.parent() with QApplication fallback, reducing global filter scope
• filesPage.py - _find_file_insert_position uses enumerate(self._model.entries) instead of rowCount() + model.index(i) + model.data() on every comparison, bypassing the SIP bridge

Motivation

py-spy profiling on the target Raspberry Pi 5 (aarch64) identified paintEvent, eventFilter, and SIP bridge calls (rowCount, model.data) as the dominant GIL consumers during idle. All changes avoid repeated allocation (paths, fonts, colors, pixmaps) and SIP crossings on the hot path. No behavioral changes.