feat(models): let people hide models they never use

[JairEsk]

A gateway provider reports 300-600 models and the picker lists every connected provider at once, so choosing a model means scrolling past hundreds you will never run. Pinning promotes a few to the top; it does nothing about the length of the list underneath. This is the other half.

Settings gains a Models section: one collapsible row per provider with a count, a search box, per-model Hide/Unhide, and Hide all / Show all scoped to whatever the search is filtering - so "gpt" + Hide all hides those matches rather than silently wiping the catalog.

Stored as a DENY-list, not an allow-list. Catalogs grow under us (a provider ships a model, a plan gains access to one), and an allow-list would withhold everything new until the user came back and ticked it. Connecting a provider still means "everything it offers", minus what you explicitly removed.

Hiding is display-only: it never touches a session's stored model, so a session already on a hidden model keeps running it. Taking something out of a menu must not silently reroute work already using it.

Details worth knowing:

• Hiding also unpins, in SQL and in the store. A pinned+hidden model would otherwise sit at the top of the very list it was removed from, and modelRows drops hidden pins too so the intermediate state can't render.
• The filter runs over all three picker sections, not just the catalog. Recents are usage history and keep listing a model after it is hidden, so a model hidden right after being used would keep reappearing under Latest.
• An empty picker now distinguishes "you hid everything" (with a link to Settings) from "models.dev failed". Telling someone their network is broken when they hid the models themselves sends them debugging nothing.
• The Settings list is windowed on a fixed row height: an expanded provider is 600 rows, which is exactly the mount cost ModelPicker already exists to avoid.
• Row actions sit on the RIGHT, in the same column as Hide all / Show all, with an always-visible label. A hover-only affordance on a 600-row list is something you have to hunt for.

hiddenModels is a Set of provider:model rather than the array shape pinnedModels uses: it is only ever asked "is THIS model hidden?", once per row over a long list, and carries no order worth preserving.

Tests: 5 checks over buildModelRows (hidden in no section, per-provider keying, a fully hidden provider drops its header, omitting the set changes nothing) and 6 against the real DB (hide unpins, idempotent, bulk replace stays scoped to its provider).

[FreddyJD]

Thank you for the change!