qt: Expand sync progress bar in status bar

[SpectrGen1]

Fixes #177.

Keep the existing single-row status bar layout and make the sync progress bar expand across the available space, so it reads like a normal left-to-right progress bar.

Root cause:

• The sync progress bar was added to the status bar without stretch, so it remained near its minimum width.
• This made the fill appear visually truncated, as if progress only moved from the far left toward the middle.

Change:

• Keep the existing single-row status bar layout.
• Give the sync progress bar an expanding size policy.
• Add the progress bar to the status bar with stretch so it occupies the available space up to the permanent right-side widgets.

This keeps the patch tightly scoped and avoids the previously discussed two-row redesign.

Manual testing:

• Ran bitcoin-qt in syncing state with a fresh signet data dir.
• Verified the progress bar spans the available single-row sync area left-to-right.
• Verified the normal non-sync state still hides the progress bar cleanly.
• Verified resize behavior remains sane as the main window grows and shrinks.
• Verified status text/icons still display sensibly.
• Spot-checked Windows styling via QT_STYLE_OVERRIDE=windows.

Visuals:

• Before/after screenshot:

• Resize behavior demo:
  https://github.com/user-attachments/assets/8781e2b5-068e-45b8-a8b8-ef345b31ed6e