Fix kanban column layout height issue

[bborn]

Summary

Re-applies the good improvements from reverted PR #478 while fixing the root cause of the kanban layout breakage.

Root cause: PR #478 showed a tmux pane preview for ALL in-progress tasks (processing/queued/blocked), which constantly consumed vertical space and broke the kanban column height calculation. The pane preview should only appear for tasks that actually need input (blocked with a permission prompt).

Key changes:

• Fix yellow highlight false positives: Rename latestPermissionPrompt → latestChoicePrompt, only match "Waiting for permission" entries (not "Waiting for user input" generic idle/end_turn scenarios)
• Replace reply mode with quick input: Replace replyActive/replyTaskID/replyPaneContent fields with a single quickInputFocused bool
• Tab key to focus input: Add QuickInput keybinding (Tab) to focus the input field — only works when selected task needs input (has a permission prompt)
• Esc/Shift+Tab to unfocus: Returns focus to the kanban board
• Preserve kanban height: Pane preview remains scoped to blocked tasks with permission prompts only, not all in-progress tasks
• Remove 'r' key hijack: The Retry handler no longer hijacks 'r' to enter reply mode

Test plan

• go build ./... compiles cleanly
• go test ./internal/ui/... all tests pass
• Select a blocked task with permission prompt → should see prompt preview with "y approve N deny tab input enter detail"
• Tab → input field focuses with "input:" label
• Type text + Enter → sent to executor
• Esc → focus returns to kanban
• Processing task without permission prompt → NO pane preview shown (kanban height preserved)
• "Waiting for user input" log → NO yellow highlight (false positive fix)

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