fix(email): a11y + mobile/UX correctness polish

[DiogoDuart3]

A batch of small, independent a11y and correctness fixes to the webmail client, verified live via a headless login + screenshot pass at 1280px and 390px before/after each change.

Mobile / correctness

• Compose FAB: a floating action button on mobile mail-list views opens the same compose dialog. Previously only reachable through the sidebar, which on mobile is a Sheet that unmounts (not just hides) while closed - 2 taps to compose. Hidden on desktop (lg:hidden) to avoid overlapping the sidebar's own persistent "New email" trigger.
• Thread toolbar: Reply-all drops its text label below md and the action row gets overflow-x-auto - the label was the one item wide enough to push the "..." menu off the right edge on a phone-width screen.
• Fixed a bare 0 rendering next to the subject on a single-message thread: totalEmails && totalEmails > 1 && ... - 0 && x evaluates to 0, and unlike false/null/undefined, React renders a numeric 0 as text. Switched to a ternary.
• Sidebar Folders/Labels section: hidden when there are no folders instead of showing a permanently empty section header.
• Sidebar account block: was showing the account email twice (once as the name-line fallback, once as the email line) for a mailbox with no display name set.
• Compose subject placeholder: "Re: Design review feedback" -> "Subject".
• Hardened redirect URLs against being baked in at build time (getAppUrl() now reads window.location.origin at runtime consistently across all redirect call sites) - the client is an SPA served same-origin, so there's no reason a redirect should ever reference a build-time value.
• Removed a stray literal + character sitting before a <button> tag in render-labels.tsx (pre-existing, not introduced by this PR) - valid JSX syntax so it doesn't break the build, but it gives a TooltipTrigger asChild two children instead of one, which Radix's Slot (React.Children.only) will throw on at runtime for any label with a visible tooltip.

Accessibility (Lighthouse-driven)

• Removed a dead Radix Collapsible wrapper around sidebar nav items that never had a matching CollapsibleContent - left a dangling aria-controls reference (aria-valid-attr-value) and put a non-<li> wrapper directly inside <ul data-sidebar="menu"> (invalid list semantics, aria-required-children).
• Added a <main> landmark around the mail app's primary content region.
• aria-allowed-attr: two *Trigger asChild sites were injecting type="button"/aria-haspopup onto bare <div>s with no interactive role - invalid. Converted to real <button>s.
• button-name: added aria-label to icon-only buttons across the mail list, thread view, sidebar, and composer (mostly reusing existing tooltip copy).
• Contrast: unscoped text-[#8C8C8C] (fails ~3.36:1 on light backgrounds) now falls back through text-muted-foreground with dark:text-[#8C8C8C] for the dark-mode accent. Darkened the #006FFE brand blue to #0062D4 for the two buttons pairing it with white text (was ~4.46:1, just under the 4.5:1 minimum). Dropped an opacity-70 utility on list-row timestamps that diluted text-muted-foreground below 4.5:1 at rest in both themes (full opacity only kicked in on hover/select).
• Fixed a real bug found while investigating a recurring Uncaught (in promise) console error (12-17x per session): the IndexedDB query-persister's shouldDehydrateQuery had dropped TanStack Query's default "only persist settled queries" check. In-flight queries still carry a live Promise field when dehydrated, which IndexedDB's structured-clone put() cannot serialize - throwing DataCloneError on nearly every persist right after login. Restored the missing status check.

Note on CI

Recent merged PRs against main (#322, #327) show Typecheck: SUCCESS / Test: FAILURE on their PR-context runs. Flagging in case it reproduces here too - doesn't look related to this change.