Planner: support rate windows and low-throughput stock goals

[ApocDev]

Problem

Most planner quantities are currently entered and displayed as per-second rates. That is mathematically consistent, but it is awkward for common planning cases:

• science targets are easier to think about as 5/min or 10/min
• slow bootstrapping goals may be 1/hr or 0.5/hr

Desired behavior

Allow users to choose a time window/unit for rate goals instead of forcing per-second input everywhere. At minimum, support seconds, minutes, and hours. The solver can continue normalizing rates internally, but the UI should preserve the user-facing unit so saved goals remain understandable.

Examples:

• 10 science/min
• 5 science/min
• 1 building/hr
• 0.5 building/hr

Questions to resolve

• Is this only a display/input unit preference, or should saved block goals store their selected rate window?
• Should the solver keep one canonical rate unit and only convert at boundaries?
• How should mixed units appear in block summaries and assistant output?

Acceptance criteria

• A user can enter and view goals in per-second, per-minute, or per-hour terms.
• Existing per-second blocks continue to load and solve unchanged.
• Saved goals remain understandable after reload, including their selected unit/window where applicable.

Note

The low-throughput "stock / coverage" use case (keep N in stock, make sure required intermediates and building materials are produced) was previously folded in here. It is now tracked on its own in #38, since it needs distinct goal semantics rather than a units preference.