Add the Explore tab screen

[AndrewG828]

Stacked PR 7 of 15. Based on #116 — merge in order.
#111 → #112 → #113 → #114 → #115 → #116 → #117 → #118 → #119 → #120 → #121 → #122 → #123 → #124 → #125

Assembles the discovery sections into a tab root: category shortcuts, a Just
For You row fronting Saved and Recently Viewed, then Daily Picks and Trending.
It is not in the tab bar yet — that comes with the navigation rework.

Explore owns a local SearchViewModel rather than the shared singleton.
Both tab roots stay mounted under the new navigation, so sharing one would mean
typing in Explore silently replaced Home's results.

SearchPanel (new) is the expanded search card, extracted rather than written
inline: Home and the category browser present the identical card, and this is
the first of the three to land. Keeping it in one place also keeps the "opaque,
not glass" decision in one place — materializing a glass layer that large in
the same frame the keyboard animates made opening search feel like a hang.

Scrolling is disabled only while the card covers the feed with no results yet.
Gating it on "is the panel open" instead — which is what the first cut did —
also freezes the search results the panel hands back, leaving you unable to
scroll past the first four.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017R8mua9xepzF3mnRETtd6W

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