feat: add AI-generated project icons

[saphid]

What Changed

The project icon picker in Settings → Project gets a Generate tab. Projects on a provider that supports image generation (Codex app-server) can describe the look they want in a small "Vibe" field, optionally tweak the auto-populated prompt under "Edit prompt", and press Generate. One Codex image-generation turn produces a single image with three icon variants; the server slices it into individual tiles, drops blank or duplicate variants, crops each kept tile to its content and centers it on a transparent square, and falls back to a single cropped icon when the model draws fewer than three distinct variants. Tiles preview in the dialog before saving; saving one stores the path on the project and renders through the existing project-favicon asset route, so the icon shows up in web and desktop alike.

Generated icons live in the server state directory (<state>/generated-icons/), not the workspace, so nothing is written to the user's project. Providers without image-generation capability (Claude, Cursor, Grok, OpenCode) see a plain "this provider doesn't support image generation" notice instead of a broken form — capability comes from a new supportsImageGeneration flag on the server provider snapshot, so gating is server-driven.

Scope: this stacks on pingdotgg#11845 and touches the web icon picker, the server (new generateProjectIcons orchestration RPC, Codex adapter, PNG slicing pipeline in apps/server/src/imageGeneration/), and the orchestration contracts. It does not touch mobile, other providers' adapters, or the checkpoint/reactor layers.

Why

pingdotgg#11845 made project icons customizable, but the palette is still a fixed set — a project's identity depends on picking whichever preset is closest. AI generation is the natural extension: the user already has an image-generation-capable provider running in the app. Keeping the model interaction server-side (one ephemeral Codex app-server thread per generation, mechanical "three variants in one image" prompt framing, defensive slicing) means the client stays dumb and the failure modes (blank variants, duplicates, symbol collages, multi-image turns) are handled in one tested place rather than per client.

Verification

• Typechecks (tsc --noEmit): @t3tools/contracts, @t3tools/client-runtime, t3 (server), web — all pass (suggestion-level diagnostics only, same as base).
• vp test run apps/server/src/imageGeneration apps/server/src/server.test.ts: 210 passed, 0 failed — includes 24 pngGrid tests covering horizontal/vertical/stacked layouts, transparent and opaque backgrounds, gap-following cuts, 2-variant cuts, blank-tile/fragment handling, duplicate rejection, single-icon fallback, sparse-artwork density crops, stacked-column reframing, and full-bleed single-icon detection.
• Hardening pass after a 4-round live bake across 60+ real generations (see head bdd4ea4b7b): fixed corner background sampling on non-square canvases, added density crops for sparse floating artwork, stacked-column reframing, and full-bleed single-icon detection; the prompt now encodes the actual render context (16px rounded-square slot on a dark sidebar).
• vp test run apps/web/src/components/settings/ProjectIconPickerDialog.test.tsx: 1 passed.
• Live end-to-end in a paired browser against a clean dev server: real Codex generation, tile previews render before saving (asset route returns 200), Save persists the absolute path to projection_projects.favicon_path, and the settings row renders the saved icon with the "Custom image" label.
• Known, unrelated: 3 CodexDriver installer-path test failures reproduce on the stacked base commit (35a4aff472) as well — not introduced here.

UI Changes

After — Generate tab with a freshly generated tile previewing before save:

After — saved generated icon rendered in the settings row:

The before state is the stacked base (pingdotgg#11845 head): the picker offers Icons/Emoji/Monogram only, with no Generate tab. No base capture is attached.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (after-only; the before state is the absence of the Generate tab in the stacked base)

Implementation used GLM (enablers/large) in T3 Code (OpenCode harness).

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for bdd4ea4.

This comment will update automatically after the next completed run.