fix: stabilize local terminal scroll sync

[qfxiongbinbin]

Summary

• batch active terminal output writes into a single animation-frame flush
• track sticky-bottom state explicitly instead of recalculating per output chunk
• debounce viewport fit and PTY resize sync to reduce cursor jumps under heavy output

Affected Areas

• local terminal rendering in ServerDeck/src/App.tsx
• terminal viewport resize synchronization for local and remote tabs

Validation

• cd ServerDeck && ./node_modules/.bin/tsc --noEmit
• manually review terminal output flow and resize handling changes

Screenshots

• N/A (behavioral fix)