Multimedia v0 — one panel that hosts many channels

TITLE: Multimedia v0 — one panel that hosts many channels BODY: IN ONE LINE: A new APP (not a new panel type) that plugs into any empty Trading Station panel and hosts MANY channels inside that ONE panel — that is the whole request. WHY IT IS GENUINELY NEW (the entire delta): The Station already gives you ONE PANEL PER FEED — the app catalog's mirrored feed panels (Community wall, Telegram public topics, Discord, X mentions), plus any platform page by path, plus #940's outside widgets by URL. Those are separate panels. THIS request is ONE PANEL THAT HOSTS MANY CHANNELS — with an "Add channel" control — so several sources live in a single unified view. Nothing else about it is new. WHAT TO BUILD (v0): 1. A catalog app "Multimedia" that mounts into any empty panel, exactly like Watchlist. 2. A CHANNEL LIST with an "Add channel" control. A channel is a URL (RSS / news), fetched SERVER-SIDE and rendered as PREVIEWS ONLY: source label · title · short preview · timestamp. Click an item → opens externally. 3. NO IFRAME IN v0. Previews only — nothing is embedded, and no third-party script runs in the platform's page. 4. The CHANNEL LIST IS SAVED INSIDE THE PANEL'S LAYOUT ENTRY. Named layouts already follow the account, so it restores across devices for free. SERVER-SIDE FETCH RULE (for user-pasted URLs): - http / https ONLY. - REFUSE private or internal addresses (SSRF guard). - SIZE and TIME CAPS on every fetch. CAPS (so the build cannot sprawl): - At most 20 channels per panel (v0). - At most 50 items kept per channel, newest first. ACCEPTANCE TESTS: 1. Add Panel → Multimedia mounts in any empty panel, like Watchlist. 2. "Add channel" accepts an RSS/news URL; its items appear newest-first as previews (label · title · snippet · timestamp). 3. Click any item → opens externally; nothing loads inside the platform. 4. The server-side fetch refuses a private/internal address and a non-http(s) scheme, and respects the size and time caps. 5. The caps hold: no more than 20 channels per panel; no more than 50 items kept per channel (newest first). 6. The channel list is stored in the panel's layout entry; reload AND another device restore it. 7. Repeated add creates no duplicate. NOT WHAT WE WANT: - NOT a new panel type — a normal app. - NOT an iframe / embedded web view in v0 — previews only (an embedded web view is a LATER channel type). - NOT a proxy browser / headless render. LATER CHANNELS (NOT v0 — appendix): private X/Telegram (one-time connection) · YouTube · Reddit · GitHub · internal platform feeds · an embedded web view. RELATIONSHIP TO EXISTING WORK: builds on feature #940 (panels as generic slots + the feed panels). This is the app that hosts many channels in one panel — not a new panel type.