fix(electron): eliminate theme flash on reload and use nativeTheme as source of truth

[AnishSarkar22]

Description

• Applied the theme before first paint using a blocking inline script in index.html, mirroring ThemeProvider's own resolution logic, so reloads no longer flash the wrong theme.
• Set the BrowserWindow's backgroundColor from nativeTheme at creation time to avoid a white flash while the window reloads.
• Persisted the theme preference to theme-prefs.json, following the same atomic-write pattern already used by window-state.ts.
• Added theme:set / theme:get-system / theme:system-changed IPC channels so the main and renderer processes stay in sync on the current theme.
• Switched system-theme resolution in the renderer to use nativeTheme (via preload) instead of matchMedia, keeping matchMedia only as a fallback for non-Electron environments like tests.

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR fixes theme flashing issues on Electron app reload by making nativeTheme the single source of truth for system theme detection. It introduces a blocking inline script in index.html that applies the correct theme before first paint, persists theme preferences to theme-prefs.json using atomic writes, and synchronizes theme state between main and renderer processes via new IPC channels (theme:set, theme:get-system, theme:system-changed). The BrowserWindow background color now matches the current theme to prevent white flashes during reload.

⏱️ Estimated Review Time: 15-30 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_local/frontend/src/lib/api.ts
2	surfsense_local/electron/src/main/theme-prefs.ts
3	surfsense_local/electron/src/preload/index.ts
4	surfsense_local/frontend/index.html
5	surfsense_local/frontend/src/components/theme-provider.tsx
6	surfsense_local/electron/src/main/index.ts
7	surfsense_local/frontend/src/features/onboarding/onboarding-page.tsx

⚠️ Inconsistent Changes Detected

File Path	Warning
surfsense_local/frontend/src/features/onboarding/onboarding-page.tsx	Unrelated copy change in onboarding text that doesn't relate to theme handling or flash elimination

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