feat(spaces): move Space-Switcher into the PillNav start slot

Repositions the switcher from its floating spot in the top right of
the workbench into the bottom-fixed PillNav so it sits with the rest
of the nav chrome. Matches how every other persistent nav control
(app switcher, AI tier, sync status) lives in the PillNav.

Mechanics:
- @mana/shared-ui PillNavigation gains a `startSlot?: Snippet` prop
  rendered inside .pill-nav-container, before AppDrawer. Generic slot
  — any host component drops in.
- (app)/+layout.svelte passes the existing <SpaceSwitcher /> as the
  snippet (authenticated only). The old .space-bar wrapper above
  <main> is removed along with its CSS.
- SpaceSwitcher trigger is restyled to match Pill conventions: pill
  radius 999px, 32px height, 0.8125rem text, tighter paddings, shorter
  name cap (7rem). Visually merges with the surrounding Pills.
- Dropdown menu flips upward (bottom: calc(100% + 4px)) because the
  PillNav is position:fixed bottom — opening downward would land
  off-screen.

Type-check: 0 errors across 7200 files.
Scope tests: 10/10 pass.
Go tests + bun tests (mana-auth): all pass.

Plan: docs/plans/spaces-foundation.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/navigation/PillNavigation.svelte

@@ -269,6 +269,12 @@
 		prependElements?: PillNavElement[];
 		/** Additional elements (tab groups, dividers) to show after nav items */
 		elements?: PillNavElement[];
+		/**
+		 * Snippet rendered at the very start of the bar, before the app
+		 * switcher. Lets the host drop a custom component (e.g. Space
+		 * switcher) into the nav without adding more dedicated props.
+		 */
+		startSlot?: import('svelte').Snippet;
 		/** Show logout button */
 		showLogout?: boolean;
 		/** Theme variant dropdown items */
@@ -355,6 +361,7 @@
 		primaryColor,
 		prependElements = [],
 		elements = [],
+		startSlot,
 		showLogout = true,
 		themeVariantItems = [],
 		currentThemeVariantLabel = 'Theme',
@@ -608,6 +615,12 @@
 		aria-label={ariaLabel}
 	>
 		<div class="pill-nav-container">
+			<!-- Host-provided start slot (e.g. Space switcher). Rendered
+				 before the app drawer so it anchors the left edge of the bar. -->
+			{#if startSlot}
+				{@render startSlot()}
+			{/if}
+
 			<!-- App Switcher (optional) -->
 			{#if showAppSwitcher && appItems.length > 0}
 				<AppDrawer

packages/shared-ui/src/navigation/types.ts

@@ -202,6 +202,14 @@ export interface PillNavigationProps {
 	prependElements?: PillNavElement[];
 	/** Additional elements to show after nav items (tab groups, dividers) */
 	elements?: PillNavElement[];
+	/**
+	 * Snippet rendered at the very start of the pill bar, before the app
+	 * switcher and any prepend elements. Lets the host drop an arbitrary
+	 * Svelte component (e.g. a Space switcher) into the bar without
+	 * adding another dedicated prop surface. Keep the rendered content
+	 * short — it's first-in-line real estate.
+	 */
+	startSlot?: Snippet;
 	/** Bottom offset from viewport bottom (default: '0px'). Use to position above other fixed bars. */
 	bottomOffset?: string;
 }