[WIP] Friends PR-2: Friend schedule dialog

[brelieu05]

Summary

Implementation of Friend Dialog and dealing with read-only + omitting features + ensuring reuse of existing calendar UI.

• FriendsStore — Holds friend schedule data separately from AppStore. Fetches via trpc.schedule.getFriendUserData and drives dialog open/close/loading state.
• friendScheduleViewSource — Read-only ScheduleViewSource (scope: 'friend') wired through a new FriendScheduleViewProvider, so ScheduleCalendar, toolbar, and event popover render friend data without touching the user's home schedule.
• Friend schedule dialog — FriendScheduleDialog → FriendScheduleView (split calendar + course list on desktop, stacked on mobile). Includes a read-only FriendSchedule course list with schedule notes.
• Minimal entry point — Header Friends button opens a popover listing friends with a View action. Unauthenticated users get the sign-in dialog. Full friends management UI (requests, search, unfriend) is deferred to PR3.
• Scope isolation — SelectedEventStore tracks selectedEventScope so event popovers don't bleed between home and friend views. Calendar toolbar hides edit actions (undo/redo, custom events, screenshot, download, clear) in readonly friend view; schedule select and finals toggle remain available.

Intentional Things Omitted from this PR and will go into PR3

• Friends popover UI (requests tab, friend search, send/cancel requests, unfriend)
• Share-schedule privacy toggles

FriendScheduleEntry.tsx is a thin placeholder UI. It is immediately deleted in PR3

Test Plan

• Home regression: Add/remove courses, switch schedules, open event popover, toggle finals — home schedule unchanged after viewing a friend
• Sign-in gate: Click Friends while logged out → sign-in dialog appears
• Friend list: Log in, open Friends popover → friends load; click View on a friend with shared schedules
• Friend dialog: Dialog opens with loading state, then calendar + course list; switch between friend's schedules via schedule select
• Readonly UI: No undo/redo, custom event, screenshot, download, or clear buttons in friend dialog; event popover shows course info without delete/color/quick search
• Scope isolation: Open schedule select or event popover in friend dialog → does not affect home calendar behind the dialog
• Empty/error cases: Friend with no shared schedules shows appropriate feedback; close dialog returns to normal home view
• Mobile: Friend dialog layout stacks calendar above course list

Issues

Depends on #1850
Closes #