feat: drag-select and copy text from main pane terminal

[sebasv]

Summary

• Adds in-app text selection for the embedded terminal: drag in the main pane to highlight a region, mouse-up auto-copies via OSC 52. Replaces the OS-level Shift+drag escape hatch (which spans the sidebar).
• Detects host clipboard support at startup from `TMUX`, `TERM_PROGRAM`, `TERM`, and emulator-specific env vars. On the first uncertain copy, surfaces a one-time hint in the sidebar with a specific remediation (tmux `set-clipboard on`, unsupported emulator name, etc.).
• Selection is cleared on tab switch, scroll, mode toggle, view change, or any keystroke that reaches the PTY, so the highlight never references stale rows.

Test plan

• Drag-select inside the main pane → highlight appears, mouse-up copies text reachable via Cmd+V in another window.
• Drag past the pane edges → selection clamps to the pane.
• Pure click (no drag) → no highlight, no copy.
• Click in sidebar / tab bar after selecting → selection clears.
• Scroll wheel after selecting → selection clears, scroll proceeds normally.
• Type into the shell after selecting → selection clears, keystrokes reach PTY.
• Run inside tmux → first copy shows `⚠ clipboard: copy may not work / tmux: needs `set -g set-clipboard on` for OSC 52`.
• Run in macOS Terminal.app → first copy shows the macOS-specific hint.
• Run in iTerm2/Kitty/WezTerm/Alacritty/Ghostty → no clipboard warning.

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