fix(fe): PageLayout grid-based layout - eliminate flex gap height calculation error

Root cause: PageLayout's flex-direction: column + gap was not counted in flex: 1 height calculations, causing children to overflow and trigger scroll.

Solution: Convert PageLayout from flexbox to CSS Grid with explicit grid-template-rows. Grid automatically accounts for gaps in row sizing.

Changes:
- PageLayout.vue: display: flex → display: grid
- grid-template-rows: auto auto auto auto 1fr auto auto
- .ks-page__content/.ks-page__aside: height: 100% → flex: 1
- .ks-page__workspace: removed flex: 1 (grid cell, not flex)

Impact:
- models-master fits viewport without page-level scroll ✓
- All screen-types layouts auto-fit with correct height propagation
- Fix applies to all pages using PageLayout

AGENTS.md v16.0: Added Layout Rule #7 (PageLayout grid requirement)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -259,6 +259,11 @@ pnpm dev
 4. **Scrollable containers:** `overflow-y: auto; min-height: 0;` (enables internal scroll without page scroll)
 5. **Grid layouts:** `align-items: start;` (NOT center/stretch) to prevent column stretching at different heights
 6. **Max-width constraint:** Wrap pages in `.page-wrapper { max-width: var(--ks-content-max); margin: 0 auto; }` to prevent 2560px+ distortion
+7. **PageLayout must use CSS Grid (NOT flexbox).** Flex + gap breaks `flex: 1` height propagation in children:
+   - ❌ **DON'T:** `display: flex; flex-direction: column; gap: var(--ks-space-2);` (gap is not counted in flex: 1 calculations)
+   - ✅ **DO:** `display: grid; grid-template-rows: auto auto auto auto 1fr auto auto; gap: var(--ks-space-2);` (gap auto-calculated)
+   - **Reason:** Grid gap is accounted for in row sizing; flex gap is invisible to flex: 1 child height calculations, causing overflow → unwanted scroll
+   - **Template rows:** header (auto) | subtitle (auto) | commandBar (auto) | summary (auto) | filters (auto) | workspace (1fr) | footer (auto)
 
 #### Page Structure Rule: NO Footers (Global or Page-Level)
 **CRITICAL:** Footers are EXCLUDED at all levels:

frontend/src/shared/ui/layouts/PageLayout.vue

@@ -39,7 +39,8 @@ const showHelp = ref(false)
   </section>
 </template>
 <style scoped>
-.ks-page { width: 100%; max-width: var(--ks-content-max); margin: 0 auto; height: 100%; flex: 1; display: flex; flex-direction: column; gap: var(--ks-space-2); }
+/* Grid-based page layout: gap is automatically calculated and doesn't break flex: 1 height propagation */
+.ks-page { width: 100%; max-width: var(--ks-content-max); margin: 0 auto; height: 100%; flex: 1; display: grid; grid-template-rows: auto auto auto auto 1fr auto auto; gap: var(--ks-space-2); }
 .ks-page__header { display: flex; align-items: center; justify-content: space-between; gap: var(--ks-space-4); flex-shrink: 0; min-height: 32px; border-bottom: 1px solid var(--ks-color-neutral-200); padding-bottom: var(--ks-space-2); }
 .ks-page__title-group { display: flex; flex-direction: column; gap: 2px; }
 .ks-page__breadcrumb { font-size: 11px; color: var(--ks-color-text-muted); line-height: 14px; }
@@ -106,8 +107,8 @@ const showHelp = ref(false)
   box-sizing: border-box;
   font-family: inherit;
 }
-.ks-page__workspace { min-width: 0; flex: 1; display: grid; min-height: 0; gap: var(--ks-space-4); }
+.ks-page__workspace { min-width: 0; min-height: 0; display: grid; gap: var(--ks-space-4); }
 .ks-page__workspace.has-aside { grid-template-columns: minmax(0, 1fr) var(--ks-aside-width); }
-.ks-page__content, .ks-page__aside { min-width: 0; min-height: 0; height: 100%; display: flex; flex-direction: column; }
+.ks-page__content, .ks-page__aside { min-width: 0; min-height: 0; display: flex; flex-direction: column; flex: 1; }
 @media (max-width: 1100px) { .ks-page__workspace.has-aside { grid-template-columns: 1fr; } }
 </style>