fix(web): restore compose markdown toolbar; slash menu is product commands only

[AlexsanderHamir]

Summary

• polish(web): compose brief uses slash commands, not a markdown toolbar #213 hid the compose brief markdown toolbar and treated / as the place for headings, lists, and quotes. The operator wanted the opposite.
• Restore menuVariant="icons" on the compose brief editor so Bold, Italic, H2, H3, lists, and code return, with the word count still on the right. menuVariant="none" remains as a unused-by-compose option.
• Compose slash menu uses slashMenu="commands": Mention a file and Ask AI only. Markdown block inserts stay in the default "all" catalog for other rich-prompt editors (polish dialog, legacy prompt field). Empty-state copy is still Press / for commands; @ file mentions are unchanged.

Test plan

• Compose brief shows the icon markdown toolbar plus 0 words (not format-buttons-hidden).
• Typing / in the compose brief lists only Mention a file and Ask AI — no Heading 2/3, lists, or Quote.
• Placeholder still reads Press / for commands; @ still mentions files.
• Other rich-prompt editors (default slash catalog) still include markdown block inserts.
• Local: .\scripts\check-web.ps1 -Group test-task-components, -Group lint, -Group test-components.

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