Fix bold font in header showing less weight (streamlit#9395)

## Describe your changes

### Context
It's been a longstanding
[issue](streamlit#4428) where
setting H1 to bold actually set the font less bold.

Example:
```
st.write("# Hello **World**")
# or st.title("Hello **World**)
```
would result in 

![image](https://github.com/user-attachments/assets/ff53a5f1-49de-41e2-b6a0-98ebea383e94)

This is because the [default font-weight of `h1` is
700](https://github.com/streamlit/streamlit/blob/develop/frontend/lib/src/theme/globalStyles.ts#L181)
and [default of `strong` or `b` is
600](https://github.com/streamlit/streamlit/blob/develop/frontend/lib/src/theme/globalStyles.ts#L313)

I consulted Jessi and we land on this expected behavior: 
**Setting text to bold in headers will NOT change its font-weight**

### This change
I proposed overriding the css font-weight for `b` & `strong` inside `h1`
to the default 700 and left font-weight for `h2+` as-is (they already
match the bolded text's weight)
Also added an e2e test for font weights of all headers.

## GitHub Issue Link (if applicable)
streamlit#4428

## Testing Plan

- Unit Tests (JS) ✅
- E2E Tests


---

**Contribution License Agreement**

By submitting this pull request you agree that all contributions to this
project are made under the Apache 2.0 license.

e2e_playwright/st_markdown.py

@@ -171,6 +171,22 @@ def draw_header_test(join_output):
 
 "---"
 
+with st.container():
+    st.text("Headers with bold syntax")
+
+    strings = [
+        "# Bold **header1**",
+        "## Bold **header2**",
+        "### Bold **header3**",
+        "#### Bold **header4**",
+        "##### Bold **header5**",
+        "###### Bold **header6**",
+    ]
+    for string in strings:
+        st.markdown(string)
+
+"---"
+
 st.latex(r"\LaTeX")
 
 st.latex(

e2e_playwright/st_markdown_test.py

@@ -31,7 +31,7 @@ def test_different_markdown_elements_in_one_block_displayed(
 
     markdown_elements = themed_app.get_by_test_id("stMarkdown")
 
-    expect(markdown_elements).to_have_count(52)
+    expect(markdown_elements).to_have_count(59)
 
     # Snapshot one big markdown block containing a variety of elements to reduce number
     # of snapshots
@@ -181,6 +181,29 @@ def test_match_snapshot_for_column_beside_widget(
     assert_snapshot(container, name="st_markdown-headers_beside_widget")
 
 
+def test_match_snapshot_for_headers_bold_text(
+    app: Page, assert_snapshot: ImageCompareFunction
+):
+    """Test that the headers with bold markdown syntex is correct."""
+    container = _get_container_of_text(app, "Headers with bold syntax")
+    assert_snapshot(container, name="st_markdown-headers_bold_syntax")
+
+    # H1 defaults to extra bold
+    h1 = app.locator("h1#bold-header1")
+    expect(h1.locator("strong").first).to_have_css("font-weight", "700")
+
+    header_ids = [
+        "h2#bold-header2",
+        "h3#bold-header3",
+        "h4#bold-header4",
+        "h5#bold-header5",
+        "h6#bold-header6",
+    ]
+    for header_id in header_ids:
+        header = app.locator(header_id)
+        expect(header.locator("strong").first).to_have_css("font-weight", "600")
+
+
 def test_help_tooltip_works(app: Page):
     """Test that the help tooltip is displayed on hover."""
     # Get the first element in the main view:
@@ -192,13 +215,13 @@ def test_help_tooltip_works(app: Page):
 
 def test_latex_elements(themed_app: Page, assert_snapshot: ImageCompareFunction):
     latex_elements = themed_app.get_by_test_id("stMarkdown")
-    assert_snapshot(latex_elements.nth(48), name="st_latex-latex")
-    expect(themed_app.get_by_test_id("stMarkdown").nth(48)).to_contain_text("LATE​X")
+    assert_snapshot(latex_elements.nth(55), name="st_latex-latex")
+    expect(themed_app.get_by_test_id("stMarkdown").nth(55)).to_contain_text("LATE​X")
 
-    assert_snapshot(latex_elements.nth(49), name="st_latex-formula")
+    assert_snapshot(latex_elements.nth(56), name="st_latex-formula")
 
-    expect(themed_app.get_by_test_id("stMarkdown").nth(50)).to_contain_text("a + b")
-    assert_snapshot(latex_elements.nth(50), name="st_latex-sympy")
+    expect(themed_app.get_by_test_id("stMarkdown").nth(57)).to_contain_text("a + b")
+    assert_snapshot(latex_elements.nth(57), name="st_latex-sympy")
 
 
 def test_large_image_in_markdown(app: Page, assert_snapshot: ImageCompareFunction):

frontend/lib/src/theme/globalStyles.ts

@@ -313,6 +313,12 @@ export const globalStyles = (theme: EmotionTheme): SerializedStyles => css`
     font-weight: ${theme.fontWeights.bold};
   }
 
+  // Override h1 font weight to default weight
+  h1 b,
+  h1 strong {
+    font-weight: ${theme.fontWeights.extrabold};
+  }
+
   // Mark
 
   mark {