feat(tui): per-section visibility for the details accordion

Adds optional per-section overrides on top of the existing global
details_mode (hidden | collapsed | expanded).  Lets users keep the
accordion collapsed by default while auto-expanding tools, or hide the
activity panel entirely without touching thinking/tools/subagents.

Config (~/.hermes/config.yaml):

    display:
      details_mode: collapsed
      sections:
        thinking: expanded
        tools:    expanded
        activity: hidden

Slash command:

  /details                              show current global + overrides
  /details [hidden|collapsed|expanded]  set global mode (existing)
  /details <section> <mode|reset>       per-section override (new)
  /details <section> reset              clear override

Sections: thinking, tools, subagents, activity.

Implementation:

- ui-tui/src/types.ts             SectionName + SectionVisibility
- ui-tui/src/domain/details.ts    parseSectionMode / resolveSections /
                                  sectionMode + SECTION_NAMES
- ui-tui/src/app/uiStore.ts +
  app/interfaces.ts +
  app/useConfigSync.ts            sections threaded into UiState
- ui-tui/src/components/
  thinking.tsx                    ToolTrail consults per-section mode for
                                  hidden/expanded behaviour; expandAll
                                  skips hidden sections; floating-alert
                                  fallback respects activity:hidden
- ui-tui/src/components/
  messageLine.tsx + appLayout.tsx pass sections through render tree
- ui-tui/src/app/slash/
  commands/core.ts                /details <section> <mode|reset> syntax
- tui_gateway/server.py           config.set details_mode.<section>
                                  writes to display.sections.<section>
                                  (empty value clears the override)
- website/docs/user-guide/tui.md  documented

Tests: 14 new (4 domain, 4 useConfigSync, 3 slash, 3 gateway).
Total: 269/269 vitest, all gateway tests pass.

ui-tui/src/__tests__/useConfigSync.test.ts

@@ -62,6 +62,53 @@ describe('applyDisplay', () => {
     expect(s.showReasoning).toBe(false)
     expect(s.statusBar).toBe('top')
     expect(s.streaming).toBe(true)
+    expect(s.sections).toEqual({})
+  })
+
+  it('parses display.sections into per-section overrides', () => {
+    const setBell = vi.fn()
+
+    applyDisplay(
+      {
+        config: {
+          display: {
+            details_mode: 'collapsed',
+            sections: {
+              activity: 'hidden',
+              tools: 'expanded',
+              thinking: 'expanded',
+              bogus: 'expanded'
+            }
+          }
+        }
+      },
+      setBell
+    )
+
+    const s = $uiState.get()
+    expect(s.detailsMode).toBe('collapsed')
+    expect(s.sections).toEqual({
+      activity: 'hidden',
+      tools: 'expanded',
+      thinking: 'expanded'
+    })
+  })
+
+  it('drops invalid section modes', () => {
+    const setBell = vi.fn()
+
+    applyDisplay(
+      {
+        config: {
+          display: {
+            sections: { tools: 'maximised' as unknown as string, activity: 'hidden' }
+          }
+        }
+      },
+      setBell
+    )
+
+    expect($uiState.get().sections).toEqual({ activity: 'hidden' })
   })
 
   it('treats a null config like an empty display block', () => {