perf: stick-to-bottom chat scroll

[SergKam]

Problem

The chat scrolled to the bottom on every appended message. Useful normally, but it made it impossible to scroll up and read earlier messages while the agent was actively streaming — each delta yanked the view back to the tail.

Fix

Auto-follow the bottom only when the user is already there. A JS `stickToBottom` flag is updated from the window scroll position:

• Within ~3 lines of the end → still "following", keep auto-scrolling.
• User scrolls further up → stop auto-scrolling; leave them to read.
• User scrolls back to the bottom → following resumes automatically.

The two modes are distinguished automatically from scroll position — no toggle, no button.

Submitting a message is an explicit "I'm engaging again" signal, so it re-engages follow and jumps to the bottom (new `force_scroll` flag on `append_user_message`), ensuring a scrolled-up user still sees their own message and the reply. History replay passes `false` so loading a tab doesn't yank a reader who scrolled up mid-load.

Scope

• `src/chat_webview.rs` — `stickToBottom` flag + passive scroll listener; `scrollToBottom()` now gated; new `forceScrollToBottom()`; `force` param on `appendUserMsg`; `force_scroll` on `append_user_message`.
• `src/ui/agent_panel/mod.rs` — live submit passes `true`.
• `src/ui/agent_panel/chat_factory.rs` — history replay passes `false`.

All other scroll call sites (streaming deltas, tool calls, system messages) inherit the behavior unchanged through the gated `scrollToBottom()`.

Checks

fmt ✅ · clippy `-D warnings` (lib+bins) ✅ · tests ✅