Fix mobile terminal scrolling, paste, and switcher focus

[karngyan]

Summary

• make touch scrollback handling cover the full usable terminal inset, including blank space around a scaled xterm surface, and cancel interrupted multi-touch drags safely
• add a first-class mobile Paste control that reads the clipboard through a user gesture and delegates to xterm so newline normalization and bracketed-paste mode remain correct
• keep the session switcher keyboard closed on mobile by focusing the dialog instead of its search field, while leaving search one tap away
• fix the mobile switcher sheet's width and scroll bounds so long session rows shrink instead of clipping the list
• enlarge mobile terminal controls and switcher rows to reliable touch targets, with horizontal overflow for very narrow screens

Audit notes

Verified in a real Chromium browser at a 342px CSS viewport:

• terminal and key-bar controls remain reachable without page overflow
• the switcher opens full-width, focus lands on the dialog, and the visual viewport does not shrink
• long session names truncate inside the sheet instead of widening/clipping it
• the list owns vertical scrolling and contains overscroll

Tests

• make lint test
• shellcheck scripts/install.sh scripts/install_test.sh
• bash scripts/install_test.sh
• targeted mobile/terminal suite: 165 tests passed
• real-browser mobile viewport inspection

The first full run hit an unrelated relay-directory log assertion once; the failing test passed 10/10 in isolation and the complete CI-equivalent command passed on rerun.