Add custom labels for individual split panes

[liewstar]

Description

Closes #11359.

• add Rename pane to the context menu opened inside a split pane
• show custom pane labels by default at the top-center of each pane
• add a per-pane Show pane label context-menu toggle
• keep labels aligned with pane geometry while splitting, resizing, and maximizing
• persist custom names and label visibility through tab recovery
• fall back to the dynamic terminal title when a custom name is cleared

Testing

• yarn eslint -f json on all changed TypeScript files
• yarn tsc -p tabby-core/tsconfig.json --noEmit
• yarn webpack --config tabby-core/webpack.config.mjs
• manually verified with a Windows x64 portable build, including context-menu rename, label visibility, session recovery, split resizing, and pane maximization

AI Usage

Choose the level of AI involvement for this PR.

• Fully vibe coded
• AI-designed, AI-coded, manually checked
• Human-designed, AI-coded
• Human-designed, human-coded (includes AI autocompletions and boilerplate gen)

_{This is not to block AI contributions but rather to speed up PR review (saves time on trying to deduce the logic behind AI hallucinations).}