fix(shell): allow attached placement for panels with fill sizing

[Louis047]

Summary

Scopes fillWidth and fillHeight surface constraints to Floating placement mode and removes the forced fallback to floating in PanelManager::openPanel.

Motivation

For plugins using fill sizing such as the Notes plugin (height = "fill"), setting the panel placement to "Attached" in settings had no effect because PanelManager::openPanel forced activePlacement = PanelPlacement::Floating. Scoping fill sizing to floating mode allows the Notes panel (and any other fill-sized panel) to attach cleanly to the bar when configured as attached, while preserving full-height sidebar behavior when configured as floating.

Type of Change

• Bug fix

Testing

• just build - compiled successfully
• just test - all 89 test suites passed
• python3 tools/i18n-check.py - verified 892 translation keys
• just lint - 563 files checked with 0 warnings / errors

Manual Coverage

• Tested on Hyprland

Screenshots / Videos

Before:

2026-08-19-21-43-24.mp4

After:

2026-08-19-21-45-23.mp4

Checklist

• This PR is ready for review.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed.
• I ran the relevant build and test commands.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• This PR does not change user-facing configuration schemas.
• This PR adds no new user-facing strings.
• I did not edit non-English translation files.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

I haven't faced any edge cases with this as I tested other panels in floating mode and they respect their panel sizes correctly without any conflicts