feat(gui): ion count and CPU time run limits in Run panel

[dhanavanthesh]

The "Ions to run" spinbox maxes out at INT_MAX (~2.1B). Load a config with max_no_ions = 5000000000 and it silently stores garbage. Type mismatch between int setter and size_t storage.

The config has max_cpu_time and the engine uses it, but there's no GUI field. Users have to edit JSON by hand.

This PR lifts the ion ceiling to 2^53 and adds a "Max CPU time (s)" spinbox.

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Changes

• Replace QSpinBox to QDoubleSpinBox (range [1, 2^53])
• Fix setMaxIons: int to quint64 with clamp at source
• Route setOptions() through setMaxIons() so JSON load is also clamped
• Define kMaxExactIons constant once, reference from both widget and driver
• Add sbMaxTime spinbox (0 = "no limit", suffix "s", step 60s)
• Wire max_cpu_time through all 6 paths in mcdriverobj.cpp
• Disable both controls while sim running; re-enable on stop
• Label as "Max CPU time (s)" engine uses CPU time, not wall time

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Note: Ion count capped at 2^53. QDoubleSpinBox uses double internally, values above 2^53 lose integer precision. Full quint64 range needs a custom text input widget.

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Tested

Ion count

Test	Result
Fresh load shows 100	✅
Load 5B ions in JSON displays 5000000000	✅
Load 1e16 ions clamps to 9007199254740992	✅
Type 2^53 + 1 clamps down to 2^53	✅
Spinbox disabled while running, re-enables on stop	✅
Revert on config load restores correct value	✅

CPU time

Test	Result
Fresh load shows "no limit"	✅
Set 120 shows "120 s"	✅
Set 0 shows "no limit"	✅
Set 10s with 4 threads stops at 2.5s wall time	✅
Set 10s with 1 thread stops at 10s wall time	✅
Disabled while running, re-enables on stop	✅
Save JSON writes Run.max_cpu_time correctly	✅
Reload JSON restores Max CPU time correctly	✅

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Closes TODO: "set limit on ion histories or time to run"

[dhanavanthesh]

@gapost ready for review. Check whenever you get a chance.