feat: Add cancel button for fast mode chat messages

[melut42]

Summary

When using the chat interface (especially in "fast mode"), users previously did not have a way to cancel an ongoing AI generation. This PR introduces a stop/cancel button that replaces the send button while the AI is streaming a response, allowing users to safely abort long-running or stuck requests.

User-Visible Changes

• UI: A new Stop button (◼) appears in place of the Send button (➤) when a message generation is in progress.
• Behavior: Clicking the Stop button aborts the ongoing AI stream immediately and resets the chat input UI to its idle state.

Implementation Details

• Added .stop-btn to index.html and implemented its styles/hover states in style.css.
• Modified messaging.js to toggle visibility between the send-btn and stop-btn based on the streaming state.
• Exposed a cancelMessage function in messaging.js which triggers the abort-message IPC call.
• Bound the click event for the new stop button in bootstrap.js.

[mo-tunn]

Thanks for adding the stop button UX — the direction is good. However, I don’t think this is safe to merge yet due to a few blocking issues:

1. Streaming state is still force-reset without guaranteed backend cancellation (high impact)
   In renderer/js/panel/bootstrap.js, the textarea focus handler forcefully resets isStreaming and re-enables controls without ensuring the in-flight request is aborted. This can allow overlapping requests and inconsistent UI/session state.

2. 60s timeout has the same race condition (high impact)
   In renderer/js/panel/messaging.js, the safety timeout sets the UI back to idle and re-enables send, but it does not guarantee the active request is actually cancelled. A slow but valid response can still be running while users send another message.

3. Multi-monitor panel positioning remains incorrect for negative coordinates (medium impact)
   In main.js:getPanelPosition(), saved x/y < 0 values are treated as invalid. This breaks common layouts where secondary displays are positioned to the left/top of the primary display.

4. PR scope is too broad for a cancel-button feature (medium impact)
   This PR includes unrelated carry-over changes (multi-monitor, AssemblyAI/PTT, prompt updates, .gitignore) from earlier commits. Please keep this PR focused to the cancel-button behavior so it can be reviewed and merged safely.

Please address the blockers above and I’ll re-review quickly.

[melut42]

Added AbortController and api.invoke("abort-message") for real backend cancellation.

Fixed race conditions by moving state resets to onAIDone and onAIError.

Synchronized UI (Send/Stop buttons) using a dedicated cancelMessage function.

Cleaned up the scope; no unrelated changes included.