feat: configurable capture settings in main window

[pungys97]

Closes #28

Summary

• CaptureSettingsManager — persists 7 tunable capture settings to userData/capture-settings.json; changes apply immediately via applyToConstants() (no restart needed)
• Advanced Settings page — accessible from a button in the main window; uses simple useState page navigation (no router dependency)
• LLM Config section — ManageKeySection and CustomEndpointSection consolidated under one section with an "or" separator to make the two options mutually exclusive and clear
• Sliders — Visual threshold, typing/scroll/click debounce, min/max activity duration, max screenshots per activity; explicit Save button (disabled until dirty) + Reset to defaults
• 13 unit tests for CaptureSettingsManager covering defaults, persist/load, reset, and applyToConstants

Test plan

• Open app → click "Advanced Settings" (bottom-right of main window)
• Adjust a slider → Save → confirm toast; reopen settings to verify value persisted
• Reset to defaults → confirm sliders return to original values
• LLM Config section shows key badge + Manage Key, with "or" / Custom endpoint below
• npm run test passes

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