fix(desktop): give the shared emoji picker a panel, and open the inbox on unread

[mattbalza]

Two unrelated desktop reports: "the emoji/reactions panel has no background and doesn't scroll", and "the inbox should open on unread".

The emoji picker

EmojiPicker renders emoji-mart into a bare unclassed <div>. Three things that make the picker usable live in .buzz-emoji-mart + useEmojiMartThemeVars + useEmojiMartStyles, and only ProfileAvatarEditor and AgentCreationPreview applied them — those render emoji-mart directly. So the component that exists to be the emoji picker was the only one without a surface.

Measured before the fix rather than assumed:

• --rgb-background unset does not make the panel transparent. It drops emoji-mart onto its own theme="auto" palette, which follows prefers-color-scheme and not Buzz's theme class. Probed live: rgb(255, 255, 255) on a white app, inside popovers that carry bg-transparent border-0 shadow-none because they assume the picker paints itself. That is the "no background".
• A wheel event over a shadow-root list is retargeted to the host, so the browser does not scroll .scroll on its own. useEmojiMartStyles installs a non-passive handler on it; without that the list is stuck on its first category. This is the half the original report called "doesn't scroll".
• w-[352px] on the host was already dead code — unlayered .buzz-emoji-mart { width: 100% } beats a layered Tailwind utility regardless of specificity. Removed with dynamicWidth; re-probed identical.

className became an override on top of the surface classes, so the two call sites that size the picker keep working.

The inbox

unreadOnly was React.useState(false) — every launch reset to "all", so the one unhandled thing had to be found by reading past everything already handled.

New useInboxUnreadOnly: defaults to unread-only, remembers the last choice in localStorage. Not sessionStorage (the list width next door is a per-window layout preference; which mail is outstanding is not). Anything but the exact string "false" reads as the default, so a corrupt value can never silently hide unread mail.

Flipping the default broke six specs that assert on specific fixture rows, most of which are read. Instead of rewriting them or faking a different default in tests, installMockBridge now seeds the preference as explicitly off — the state of a user who already switched it — with seedInboxUnreadOnly: false to opt out. One new spec opts out and covers the shipped default.

Test plan

• node --test desktop/src/features/home/useInboxUnreadOnly.test.mjs — 6/6
• desktop/tests/e2e/emoji-picker-surface.spec.ts — 3/3, two proven red before the fix
• desktop/tests/e2e/inbox-unread-default.spec.ts — 3/3
• emoji + reaction smoke batch — 27/27
• inbox / drafts / unread / reminder / project-inbox / home-collapsed batch — 59/59
• npx tsc --noEmit clean; biome check clean on the changed files