fix: preserve scroll position when loading older messages

[0xSolarPunk]

Summary

• stop the chat mount autoscroll effect from retriggering when older messages are prepended
• preserve the viewport anchor after load-more waits for Svelte to flush rendered messages
• guard anchor restoration if the user switches chats while older messages are still loading
• add regression coverage for anchor preservation and stale chat-switch behavior

Review fix

• Rebased onto current cfal/garcon main.
• Added a selected-chat guard around the async anchor restore path so a completed older-message load cannot mutate the next chat's scroll position after navigation.

Validation

• bun run --cwd web test -- src/lib/chat/tests/conversation-scroll-controller.test.ts src/lib/chat/tests/conversation-feed-items.test.ts
• bun run --cwd web i18n:compile
• bun run check
• bun run test
• timeout 30s bun run start --port 0

[0xSolarPunk]

@codex review