fix(flow-chat): reduce excessive input box and footer heights for short windows

[xielixing]

Issue #1580

The input box and streaming output area have height design problems. When the chat pane is short (e.g., when the bottom terminal is expanded), the input box min-height values (76px box, 56px input-area) are too large, causing excessive blank space. The footer spacer geometry (NORMAL_INPUT_BLOCK_SAFE_PX=96) forces a 124px footer reservation, consuming too much vertical space and leaving very little room for streaming output.

Changes

• ChatInput.scss: Reduce __box\ min-height 76px -> 56px, multi-line __input-area\ min-height 56px -> 28px, base __input-area\ min-height 56px -> 28px
• flowChatScrollLayout.ts: Reduce \NORMAL_INPUT_BLOCK_SAFE_PX\ 96 -> 64
• VirtualMessageList.scss: Reduce footer fallback height 120px -> 92px, min-height 72px -> 56px
• ScrollToLatestBar.scss: Reduce default height 114px -> 98px, content bottom 90px -> 74px

The \FALLBACK_INPUT_BLOCK_ACTIVE_PX=96\ is kept unchanged to prevent the white-screen scroll clamp issue when input collapses to capsule mode. Capsule mode is not affected (already overrides __input-area\ to \min-height: 0).

Validation

• TypeScript transpile: OK
• SCSS brace balance: OK
• 4 files changed, 9 insertions(+), 9 deletions(-)