explore(desktop): anonymous app-shell home on first open (Discord model)

[thomaspblock]

Exploration: anonymous app-shell home on first open

Second exploration branch for the onboarding flip, requested in buzz-onboarding. Builds on #5976 (explore/discord-style-landing).

The idea (Discord model): a fresh install opens anonymously into something that looks like the app — a ghost community rail + sidebar skeleton on the left, and a community-discovery home in the main pane — instead of a centered onboarding corridor or an agent greeting.

What changed

• New AnonymousShellLanding: full-bleed mock app frame. Left: ghost rail (compass home, community skeletons, +) and sidebar (search, Inbox/Pulse/Projects/Agents, channel + DM skeletons, "Guest" identity card). Main pane: "Find your community on Buzz" hero, featured community cards with one-click Join, and quiet below-the-fold options (classic identity/agents corridor, key import).
• The discover page now escapes the onboarding shell entirely — no chartreuse welcome chrome, no max-w-[1040px] centered stack; renders h-dvh app-frame layout in the normal app theme.
• One-click Join unchanged from explore(desktop): discovery-first onboarding landing (Discord-style) #5976: identity persisted silently, communityOnboarding.start() connects to the chosen relay; backup/harness/agent config deferred to post-join.
• Replaces DiscoveryLanding (superseded by this layout); spec renamed to anonymous-shell-shot.spec.ts with rail/sidebar assertions.

Verification (local, commit a737d62)

• pnpm typecheck, pnpm check, unit suite: green (4954 pass / 0 fail).
• anonymous-shell-shot.spec.ts e2e: passes (shell renders, corridor reachable and returns, one-click join lands in the app).
• Onboarding-adjacent smoke specs (identity-lost, onboarding-backup, key-import-reveal, identity-key-help): 13 failures locally, but the identical failure set reproduces on the base commit 3ddaa0d (explore(desktop): discovery-first onboarding landing (Discord-style) #5976, where CI shows the same smoke shards failing) — pre-existing on the exploration base, not introduced here.

Exploration only — not intended to merge as-is.

[thomaspblock]

Screenshots from the anonymous-shell-shot.spec.ts run (1280×960):

1. Anonymous shell home — first open, no identity: ghost rail + sidebar, discovery in the main pane.
2. Advanced setup corridor — reachable one click below the fold; back returns to the shell.
3. After one-click Join — identity created silently, landed in the app.

[themiguelamador]

Three blocking/polish findings on the current exploration head:

• The landing escapes the neutral onboarding theme but keeps Card variant="textured" at its default light texture. On a fresh install following a dark system theme, the cards become bright white powder under light text; names, counts, and descriptions are effectively unreadable.
• At the supported 800px minimum window, the fixed 296px ghost chrome leaves too little main-pane width while the viewport-wide sm:grid-cols-2 still forces two 224px-minimum cards. Content truncates and the textured surfaces crowd each other.
• The newly visible ghost-sidebar copy has eight WCAG AA contrast failures in dark mode (ratios down to 2.8:1).

I fixed all three in signed commit fbd86df8c and published the proposed patch here:
https://github.com/Complear/buzz/tree/review/pr-5987-fix

The patch selects the texture from useTheme().isDark, delays the two-column layout until lg, raises decorative-copy contrast, and adds a dark 800x500 E2E regression for texture choice, stacking, and horizontal overflow.

Verification: E2E build; onboarding E2E 2/2; desktop typecheck; desktop unit tests 4954/4954; focused Biome; file-size/text-size guards; interactive 800x500 dark browser check; post-fix axe cleared all PR-introduced contrast violations (only the pre-existing empty document title remains).