Add pinned notes feature

[Copilot]

Adds ability to pin notes for quick access. Pinned notes appear in a dedicated page accessible from the main menu and link back to their original day.

Changes

Data Model

• Added IsPinned boolean to DataPoint with EF Core migration
• Pin state persists via existing page lifecycle hooks (navigation, deactivation, disposal)

UI Components

• Pin button in note view (color indicates pin state)
• PinnedNotesPage lists all pinned notes, ordered by date then creation time
• "Pinned notes" menu item in main dots menu
• Clicking pinned note navigates to origin day with scroll-to-note via query param

Implementation Notes

• SQLite DateTimeOffset ordering done client-side to avoid query translation issues
• Follows existing patterns: UI changes saved via page SaveState() lifecycle
• 3 new tests covering pin/unpin, display, and empty state

// Pin button in DataPointView.razor
<MudIconButton Icon="@Icons.Material.Rounded.PushPin"
               Color="@(Point.IsPinned ? Color.Primary : Color.Default)"
               OnClick="TogglePin" />

// Navigation from pinned note
NavigationManager.NavigateTo($"/{date:yyyyMMdd}?scrollToNote={note.Guid}");

Original prompt

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This section details on the original issue you should resolve

<issue_title>Pinned notes</issue_title>
<issue_description>Show a pin icon button next to the edit button. Clicking that will mark it as pinned.

Pinned notes will show up in a separate "Pinned notes" page found in the dots menu.
Clicking a note in there will take you back to that day and scroll to that note.

Pins are saved between launches.
Pins are ordered by the date of the note itself.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Pinned notes #8

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