[feat][#963] VSCode Plugin Equal-Width Tabs with Overflow Protection

[were]

[feat][#963] VSCode Plugin Equal-Width Tabs with Overflow Protection

Implement equal-width tabs for the VSCode plugin's unified view to improve
visual harmony and professional appearance.

Changes

vscode/src/view/unifiedViewProvider.ts:

• Add flex: 1 to distribute equal width to all three tabs (Plan, Worktree, Settings)
• Add text-align: center to center tab labels
• Add white-space: nowrap to prevent text wrapping
• Add overflow: hidden and text-overflow: ellipsis for graceful overflow handling

vscode/src/view/unifiedViewProvider.md:

• Document tab styling behavior including flexbox equal-width distribution
• Document overflow protection strategy for narrow sidebars

tests/vscode/test-tab-styling.sh:

• New visual regression test to verify all required CSS properties are present

Success Criteria

• ✅ flex: 1 distributes equal width to all three tabs
• ✅ text-align: center centers tab labels
• ✅ white-space: nowrap prevents text wrapping
• ✅ text-overflow: ellipsis truncates with "..." when space is insufficient
• ✅ All new CSS properties are documented in interface docs
• ✅ Visual regression tests pass

Test Results

• tests/vscode/test-link-detection.sh: 14 passed, 0 failed
• tests/vscode/test-step-progress-parser.sh: 7 passed, 0 failed
• tests/vscode/test-tab-styling.sh: passed

Build verified: make vscode-plugin compiles successfully with no errors.

Issue 963 resolved

closes #963