Display loading and save progress bars

[mondalaci]

Summary

• Add a grayscale progress bar below the spinning agent icon on the startup loading screen, driven by real config read progress over IPC.
• Add a left-to-right progress fill on the Save to keyboard button during saves, with monotonic progress that stays full through the Saved! state.
• Report byte-based transfer progress from the main process and rebalance phase allocation so the bars track actual work instead of jumping at the end.

Closes #2355

Test plan

• Start Agent with a connected keyboard and confirm the loading screen bar advances smoothly to 100% before the app opens.
• Change a setting and click Save to keyboard; confirm the button fill tracks the save and stays full until Saved! disappears.
• Verify progress in both light and dark themes.

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