Add panel-local shortcuts for switching tabs

[mobrava]

Description

Command+1 selects History and Command+2 through Command+9 select the first eight pinboards, in the order the tabs are drawn. The shortcuts are handled by the panel's own key monitor, so they work while the search field has focus, do nothing when the panel is closed, and register no global hotkeys. Nothing is added to Settings > Shortcuts as a recorder, since these are fixed, but the mapping is documented there and in each tab's tooltip.

Mouse clicks, Escape, and pinboard create/delete now go through one switching path (PanelController.selectTab) that drops a stale Quick Look and clears the previous tab's keyboard target before the new tab renders, so a fast Return cannot paste a clip from the tab that was just left.

PanelTab moved to Clipbara/Utilities/PanelTabShortcut.swift next to the mapping it belongs with. A new unhosted ClipbaraTests target covers the key mapping, and CI runs it.

Closes #8

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactoring (no functional changes)
• Documentation update
• Other (please describe):

How to Test

bash scripts/test-tab-shortcuts.sh

The script runs the unit tests, builds Debug, and relaunches the app. Then, with the panel open:

1. Press Command+1, Command+2, and Command+3, and check the cards, not just the tab highlight.
2. Switch while the search field has focus, while Quick Look is open, and while a pinboard dialog is open. The query survives, the preview closes, and the dialog keeps its input.
3. Confirm a missing tab number, Command+0, and Command+Shift+number do nothing, and that the shortcuts are inert while the panel is closed.

docs/testing/issue-8.md has the full matrix.

Checklist

• I have tested this change on macOS 15+
• The project builds without warnings (xcodegen generate && xcodebuild)
• I have followed the existing code style and conventions
• I have updated documentation if needed

[mobrava]

Merged into main as 846ee60. Squashed locally rather than through the button so the commit author stays consistent with the rest of the history.