Fix UI hangs, cancellation, history limits, and robustness issues

[ExTV]

• run_bash_tool: capture output via temp file instead of command
  substitution, so commands that leave background children no longer
  freeze the UI forever; add timeout -k hard-kill fallback
• Keep the UI responsive during API calls and tool runs: curl and tool
  commands run as background jobs while events are drained, Send becomes
  a Stop button that cancels the run, and quit/collapse/drag/Copy/Speak
  keep working; cancelled tool calls still get tool replies so the
  conversation history stays valid
• Build request bodies via stdin/files instead of --argjson/-d argv,
  which failed with 'Argument list too long' past the kernel's 128KiB
  per-argument limit; surface real curl errors instead of a generic
  'Network error'
• Trim conversation history between turns (keeping the system prompt and
  never orphaning tool results) so context and cost stay bounded
• Handle the destroy event so the script exits when Android kills the
  overlay, and update screen bounds on config changes
• Clamp dragging to the screen so the bubble can't be lost off-screen
• Sanitize control characters (\r, ANSI escapes, the protocol's RS
  delimiter) out of transcript text and cap its length, fixing protocol
  corruption and progressively slower updates
• Ensure assistant messages always carry content or tool_calls; validate
  API responses are JSON before parsing
• Preserve '=' characters when parsing config values
• Check for required tools at startup; warn if termux-api is missing
• Default model: deepseek-chat (documented alias); still overridable
• README: document Stop, model override, and name the GPL-3.0 license

Co-Authored-By: Claude Fable 5 noreply@anthropic.com