[Bug] Settings screen renders blank after process recreation due to uninitialized CameraSystemConstraints

When the app process is terminated while in the background on the Settings screen (e.g., via OS memory reclamation, developer options "Don't keep activities", or when revoking a permission in System Settings), returning to the app restores `SettingsScreen` from the navigation backstack, but all settings items fail to render. The user is presented with a blank screen showing only the top header ("Settings") and the back button.

Navigating back to `PreviewScreen` and returning to `SettingsScreen` recovers the UI.

## Steps to Reproduce
1. Open the app and navigate to the **Settings** screen.
2. Background the app.
3. Terminate the app process in the background (e.g., revoke any permission in Android System Settings, enable "Don't keep activities" in Developer Options, or run `adb shell am kill com.google.jetpackcamera`).
4. Reopen the app from Recents.
5. Observe the `SettingsScreen`.

## Expected Behavior
`SettingsScreen` should render all available settings options upon process recreation, or display a loading indicator while fetching hardware constraints. Furthermore, if a critical permission (such as `CAMERA`) was revoked while backgrounded, the app should guard navigation and route back to the permissions onboarding flow.

## Actual Behavior
The settings list is completely empty; only the top app bar is drawn.

## Root Cause
1. **Coupled Constraints Lifecycle**: `SettingsViewModel` combines with `constraintsRepository.systemConstraints.filterNotNull()` to construct `SettingsUiState.Enabled`.
2. **In-Memory Singleton Reset**: `SettableConstraintsRepositoryImpl` is an in-memory singleton whose `_systemConstraints` initializes to `null` on new process creation.
3. **Bypassed Preview Initialization**: `constraintsRepository.updateSystemConstraints(...)` is currently only called inside `PreviewViewModel.init` when the viewfinder is active. Because Compose Navigation restores `SettingsScreen` as the top backstack destination after process death, `PreviewScreen` and `PreviewViewModel` never run.
4. **Permanent Flow Suspension**: `constraintsRepository.systemConstraints.filterNotNull()` suspends indefinitely waiting for a non-null emission. `SettingsViewModel.settingsUiState` remains stuck on its initial `SettingsUiState.Disabled` state, which renders an empty `Column` in `SettingsScreen`.
5. **Missing Mandatory Permission Guard**: If `CAMERA` permission was revoked during process termination, `SettingsScreen` has no top-level guard checking whether baseline permissions are still held.

## Proposed Solutions
1. **Decouple Constraint Discovery from the Preview Lifecycle**:
- Allow `ConstraintsRepository` (or `CameraSystem`) to query camera characteristics directly via `ProcessCameraProvider` on demand in the background. Querying camera characteristics (lenses, supported resolutions, flash, video capabilities) does not require active camera streaming or runtime camera permissions.
2. **Handle Loading / Fallback States in Settings**:
- Add a proper `Loading` state or display default/fallback constraints in `SettingsViewModel` rather than hanging indefinitely on an empty `SettingsUiState.Disabled`.
3. **Root Navigation Permission Guard**:
- On process restoration, verify baseline mandatory permissions (`CAMERA`). If missing, clear the backstack and route to `PermissionsRoute` to prevent child screens from restoring in an unexecutable state.