Refactor: template theme.qss from theme_colors.py and strip inline styles from .ui files

[pecomyint]

Summary

Currently colors and font sizes are defined in three places:

• theme_colors.py — Python constants (the source of truth for dynamic styles)
• theme.qss — hardcoded hex values that must manually match theme_colors.py
• .ui files — inline styleSheet properties with hardcoded hex values

This means a color change requires updating up to 3 locations.

Proposed solution

1. Template theme.qss — Replace hardcoded hex values with @VARIABLE@ placeholders (e.g. @SUCCESS@, @FONT_BODY@). Update configure_app() to substitute them from theme_colors.py before calling setStyleSheet().

2. Strip inline styleSheet from .ui files — Move those rules into theme.qss (with templated placeholders). Keep .ui files as pure layout.

3. Single source of truth — Change a color or font size in theme_colors.py → flows to .qss, Python code, and all dialogs automatically.

Files affected

• src/dashpva/gui/__init__.py — update configure_app() to do template substitution
• src/dashpva/gui/theme.qss — replace ~30 hardcoded hex values with @PLACEHOLDERS@
• ~8-10 .ui files with inline styleSheet properties:
  • launcher/launcher.ui, workflow/workflow.ui, dashpva.ui, settings_dialog.ui, workbench.ui, metadata_converter.ui, hkl_viewer_window.ui, update_dialog.ui, install/update_dialog.ui

Risk

Medium surface area — every .ui file with styles gets touched. Needs manual visual testing of each dialog after the change.

Context

Color centralization into theme_colors.py was completed in dev-peco (ROI colors, log colors, status colors, launcher contrast). This issue covers the remaining architectural step to make .qss and .ui files also consume from that single source.