style(sidebar): swap expand chevrons for +/− indicators on nav sections

Streamlit's default sidebar section header uses a Material Symbols
expand_more chevron — three different icons (chevron down, chevron up,
sometimes a plain triangle) depending on version, all of which felt
inconsistent with the rest of the chrome.

Hide the built-in icon (svg / material-symbols span — covered with
multiple selectors for cross-version durability) and render our own
glyph as a right-aligned pseudo-element on the section-header button,
keyed off the standard ARIA aria-expanded attribute:
- collapsed → "+"
- expanded  → "−" (U+2212, visually balanced with +)

Hover deepens the indicator color to match the surrounding nav-link
hover treatment.

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

src/gui/components/_legacy.py

@@ -335,6 +335,55 @@ body, .stApp {
     box-shadow: none !important;
 }
 
+/* ---------- Section header expand indicator ----------
+   Swap Streamlit's default ``expand_more`` chevron in each sidebar
+   section header for a soft ``+`` / ``−`` pair. ``+`` when collapsed,
+   ``−`` (U+2212, visually balanced with +) when expanded.
+
+   We hide any built-in icon Streamlit injects (svg or Material
+   Symbols span — exact element differs by Streamlit version) and
+   render our own glyph as a right-aligned pseudo-element keyed off
+   ``aria-expanded``. The button itself gets a touch of right padding
+   so the label doesn't run into the indicator. */
+[data-testid="stSidebarNavSectionHeader"] {
+    position: relative !important;
+}
+[data-testid="stSidebarNavSectionHeader"] button {
+    width: 100% !important;
+    padding-right: 22px !important;
+    background: transparent !important;
+    border: none !important;
+}
+[data-testid="stSidebarNavSectionHeader"] button svg,
+[data-testid="stSidebarNavSectionHeader"] button [data-testid="stIconMaterial"],
+[data-testid="stSidebarNavSectionHeader"] button span.material-symbols-rounded,
+[data-testid="stSidebarNavSectionHeader"] button span.material-symbols-outlined,
+[data-testid="stSidebarNavSectionHeader"] button span.material-icons,
+[data-testid="stSidebarNavSectionHeader"] button span.material-icons-outlined {
+    display: none !important;
+}
+[data-testid="stSidebarNavSectionHeader"] button[aria-expanded]::after {
+    position: absolute;
+    right: 8px;
+    top: 50%;
+    transform: translateY(-50%);
+    font-size: 16px;
+    font-weight: 400;
+    line-height: 1;
+    color: var(--ink-tertiary);
+    transition: color 0.15s ease;
+    pointer-events: none;
+}
+[data-testid="stSidebarNavSectionHeader"] button[aria-expanded="false"]::after {
+    content: "+";
+}
+[data-testid="stSidebarNavSectionHeader"] button[aria-expanded="true"]::after {
+    content: "\2212";
+}
+[data-testid="stSidebarNavSectionHeader"] button:hover::after {
+    color: var(--ink) !important;
+}
+
 /* Inline + block code → mono with subtle accent chip. theme.py owns
    the family + size; this layer adds the warm-fill background. */
 [data-testid="stMarkdownContainer"] code {