fix(herdr): Honor tab display base in new-tab suggestions

[Unique-Divine]

Fix Herdr new-tab suggestions for zero-based indexing

Make both tab-creation dialogs suggest 6 when the workspace has tabs indexed 0 through 5, instead of suggesting 7.

• Closes bug(herdr): New tab dialog ignores zero-based indexing #57

Rationale

The keyboard dialog and tab context menu hard-code a one-based suggestion even when the tab bar uses zero-based indexes. Both dialogs should use the existing method tab_display_index_base() so the suggested name follows the same presentation rule. Default and mixed bindings retain one-based suggestions.

Key Changes

1. Use the configured display base in both dialogs, counting only tabs in the target workspace.
2. Cover both creation paths with real snapshots, including a context-menu target outside the focused workspace. Accepting the suggestion still sends label: None; editing it sends a custom name. Public tab IDs and numbering retain their upstream meaning.
3. Record tab-creation suggestions in the vendored fork's local-change notes.

The full local check encounters two unrelated clipboard tests that read the Windows clipboard before their fake X11 commands on WSL. The remaining suite passes with those two tests excluded. Windows cross-lint is unavailable because this machine lacks the SDK configuration.