fix(FN-2861): harden planning session recovery and retry UX
- Add planning and subtask retry routes with session-lock checks and proper API error mapping
- Improve planning session execution with timeout/abort handling, stop-generation support, and resilient stream catch-up behavior
- Update Planning Mode modal to handle reconnects, retry-from-error flow, stop action, and cross-tab session state synchronization
- Expand dashboard tests for planning routes, planning session behavior, and PlanningModeModal retry/error coverage
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