Add batch generation support

[Steve Castellotti (castellotti)]

Summary

• Adds batch generation to GenSpace: run the same prompt N times sequentially for both image and video modes
• Batch dropdown offers preset counts (100×, 50×, 20×, 15×, 10×, 5×, 4×, 3×, 2×, 1×) plus a free-entry custom input (digits only, max 5 chars, min 1)
• While a batch is running, the trigger shows the remaining count (e.g. 9× after the first of 10); the dropdown's selected item always reflects the original chosen count so no temporary values appear as options

Screenshot

Test plan

• Generate a batch of videos and confirm each asset shows the correct prompt when clicked
• Verify the batch counter decrements correctly and resets to the chosen count when done
• Confirm custom batch count input accepts only digits, rejects non-numeric characters, caps at 5 digits, and requires value ≥ 1
• Confirm all preset batch sizes work end-to-end
• Verify single generation (1×) is unaffected

Checks

TypeScript (pnpm typecheck:ts):

> tsc --noEmit
(no errors)

Python / Pyright (via test_pyright.py):

PASSED tests/test_pyright.py::test_pyright_has_no_errors_or_warnings

Backend tests (pnpm backend:test):

======================== 262 passed in 60.75s (0:01:00) ========================