[Frontend]: Add skeleton loading for micro-app fetching, and loading feedback for sign-out

[MohamadNazik]

Purpose

Closes #97

Several async operations gave the user no feedback while in flight: fetching the micro-apps list (both in the Store and My Apps screens) and signing out. Users had no indication anything was happening, and could still interact with the tab bar / navigate mid-logout.

Goals

• Show skeleton loading placeholders while micro-app data is being fetched, on both the Store screen (available apps) and the My Apps screen (installed apps).
• Show a loading spinner on the Sign Out button and block tab bar interaction while sign-out is in progress.

Approach

• Added Skeleton-based ListItemSkeleton/ListItemSkeletonList (row-style, for the Store screen's list layout) and WidgetSkeleton/WidgetSkeletonGrid (grid-style, for the My Apps screen's icon-grid layout) components.
• useStore.ts: fixed a bug where loadMicroAppDetails() was called without await, so isLoading was reset before the fetch actually completed — the skeleton had no real time window to render regardless of fetch duration.
• useMyApps.ts: added an isLoading state around the initial fetch, which previously had no loading state at all.
• authSlice.ts: added isLoggingOut state + setLoggingOut action; performLogout.ts dispatches it at the start/on error of the logout thunk (success path resets via the existing resetAll()).
• profile.tsx: Sign Out button shows a spinner + "Signing Out..." and is disabled while logging out.
• (tabs)/_layout.tsx: renders an absolutely-positioned transparent overlay over the tab bar while isLoggingOut is true, blocking all touches there regardless of the tab bar's internal press-handling. (An earlier attempt using tabBarButton's disabled prop didn't reliably block navigation in testing, so the overlay is the actual guard.)

No UI changes to the native splash/app icon or other unrelated screens.

User stories

As a user, when I open the Store or My Apps screen, I see a loading skeleton instead of a blank screen while data loads. When I sign out, I see clear feedback that it's in progress and can't accidentally navigate away mid-process.

Release note

Added skeleton loading states for micro-app fetching (Store and My Apps screens) and loading feedback (spinner + disabled tab bar) during sign-out.

Automation tests

• Integration tests: Verified manually via an EAS development build — confirmed skeletons render during fetch, sign-out spinner/disabled state work, and tab bar is correctly blocked during logout.

Security checks

• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Test environment

Tested on a physical Android device via an EAS development build.