Dedent ui.mermaid content like ui.markdown does

[falkoschindler]

Motivation

ui.markdown already auto-dedents its content via remove_indentation, but ui.mermaid only stripped outer whitespace.
Identical triple-quoted strings therefore behaved differently between the two elements.

The discrepancy becomes a hard error with Mermaid's YAML front-matter (e.g. displayMode: compact for compact-mode Gantt charts), which requires --- markers at column 0.
Any leading indentation makes Mermaid bail out with "Diagrams beginning with --- are not valid. If you were trying to use a YAML front-matter, please use un-indented --- blocks".

Refs discussion #6011.

Implementation

• Promote remove_indentation from nicegui/elements/markdown.py to nicegui/helpers/strings.py and re-export it via nicegui.helpers.
  It is no longer markdown-specific now that ui.mermaid uses it as well, and lifting it removes the cross-element dependency from mermaid → markdown.
• Update all existing call sites (code, markdown, restructured_text, tests/test_vbuild.py, three website files) to import from the new location.
• Use remove_indentation inside Mermaid._handle_content_change, replacing the previous content.strip() calls.
  Since remove_indentation already drops leading whitespace-only lines and produces no leading whitespace on the first line, no extra .strip() is needed; Mermaid is fine with any trailing whitespace.
• Add a regression test test_mermaid_with_yaml_frontmatter that places a YAML front-matter block inside an indented triple-quoted string and verifies the chart renders without a syntax error.

This is not a breaking change in practice: Mermaid is whitespace-insensitive at the start of lines for every diagram type, so removing a uniform leading indent is a no-op for existing diagrams.
The four demos in mermaid_documentation.py continue to render identically.

Progress

• The PR title is a short phrase starting with a verb like "Add ...", "Fix ...", "Update ...", "Remove ...", etc.
• The implementation is complete.
• This PR does not address a security issue.
• Pytest has been added.
• Documentation is not necessary.
• No breaking changes to the public API.

[evnchn]

Refactor looks sound (trusting your AI agent). On my end I watch for the empirical results:

Please merge only after "Claude, is it safe to dedent all Mermaid diagrams automatically" yields a Yes.

[falkoschindler]

@evnchn Yes, Claude couldn't come up with a counter example where existing Mermaid code would break after this change. 👍🏻