refactor(ui): unified bottom-stack container for PillNav, QuickInput, TagStrip

Replace 4 independent position:fixed elements with one flex container that
stacks them naturally from bottom to top. Elements push each other
automatically — no more hardcoded offsets or z-index conflicts.

Stack order (bottom → top):
1. PillNavigation (collapsible)
2. TagStrip (togglable)
3. QuickInputBar + toggle button row

Shared-UI changes:
- PillNavigation: add positioning='fixed'|'static' prop
- QuickInputBar: add positioning='fixed'|'static' prop
- TagStrip: add positioning='fixed'|'static' prop
- All default to 'fixed' for backward compatibility

Layout changes:
- Wrap all bottom elements in .bottom-stack (position:fixed, flex-column)
- Remove hardcoded bottomOffset calculations
- Toggle button is now inline next to QuickInputBar (not separately positioned)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -228,6 +228,8 @@
 		onToggleTheme?: () => void;
 		/** Whether dark mode is active */
 		isDark?: boolean;
+		/** Use 'static' when inside a flex container (bottom-stack pattern). Default: 'fixed'. */
+		positioning?: 'fixed' | 'static';
 		/** Whether navigation is collapsed */
 		isCollapsed?: boolean;
 		/** Called when collapsed state changes */
@@ -318,6 +320,7 @@
 		onLogout,
 		onToggleTheme,
 		isDark = false,
+		positioning = 'fixed',
 		isCollapsed: externalCollapsed,
 		onCollapsedChange,
 		languageItems = [],
@@ -403,6 +406,7 @@
 {#if !(externalCollapsed ?? false)}
 	<nav
 		class="pill-nav"
+		class:pill-nav-static={positioning === 'static'}
 		style="{primaryColor
 			? `--pill-primary-color: ${primaryColor};`
 			: ''}--pill-nav-bottom: {bottomOffset}"
@@ -828,6 +832,12 @@
 		container-name: pillnav;
 	}
 
+	.pill-nav-static {
+		position: relative;
+		bottom: auto;
+		z-index: auto;
+	}
+
 	.pill-nav-container {
 		display: flex;
 		align-items: center;

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

@@ -34,6 +34,8 @@
 		createHref?: string;
 		/** Whether the filter strip below is visible (adjusts bottom position) */
 		aboveFilterStrip?: boolean;
+		/** Use 'static' when inside a flex container (bottom-stack pattern). Default: 'fixed'. */
+		positioning?: 'fixed' | 'static';
 	}
 
 	let {
@@ -48,6 +50,7 @@
 		showCreateButton = true,
 		createHref,
 		aboveFilterStrip = false,
+		positioning = 'fixed',
 	}: Props = $props();
 
 	const resolvedCreateHref = $derived(createHref ?? managementHref + '?new=true');
@@ -63,7 +66,11 @@
 	}
 </script>
 
-<div class="tag-strip-wrapper" class:above-filter-strip={aboveFilterStrip}>
+<div
+	class="tag-strip-wrapper"
+	class:above-filter-strip={aboveFilterStrip}
+	class:tag-strip-static={positioning === 'static'}
+>
 	<div class="tag-strip-container">
 		<!-- Clear Filter Button (always rendered to prevent layout shift) -->
 		<button
@@ -140,6 +147,12 @@
 		left: 0;
 		right: 0;
 		z-index: 49;
+	}
+
+	.tag-strip-static {
+		position: relative;
+		bottom: auto;
+		z-index: auto;
 		display: flex;
 		flex-direction: column;
 		align-items: stretch;