chore(deps): consolidate 15 Dependabot updates + Tailwind v4 migration

[sfph]

Summary

• Merges all 15 open Dependabot PRs (chore(deps): bump actions/upload-artifact from 4 to 7 #37–chore(deps-dev): bump tailwindcss from 3.4.19 to 4.2.2 in /mycelium-frontend #51) into a single testable branch
• Migrates the frontend from Tailwind CSS v3 to v4 (required by the tailwindcss bump to ^4.2.2)
• Regenerates uv.lock to match the updated pyproject.toml constraints

GitHub Actions

Dependency	Change
actions/upload-artifact	4 → 7
actions/download-artifact	4 → 8
actions/stale	9 → 10
astral-sh/setup-uv	4 → 7
docker/build-push-action	6 → 7

Backend (fastapi-backend/pyproject.toml)

Dependency	Change
fastapi	<0.116 → <0.136
litellm	1.82.3 → 1.82.6
sentence-transformers	<3 → <6
transformers	<4.40 → <5.5
ruff	<0.12 → <0.16

Frontend (mycelium-frontend/package.json)

Dependency	Change
next	^15.5.14 → ^16.2.1
tailwind-merge	^2.6.1 → ^3.5.0
typescript	^5.9.3 → ^6.0.2
@types/node	^22.19.15 → ^25.5.0
tailwindcss	^3.4.0 → ^4.2.2

Tailwind v4 migration (fix for breaking change)

• postcss.config.mjs: switched from tailwindcss plugin to @tailwindcss/postcss
• globals.css: replaced @tailwind base/components/utilities with @import "tailwindcss" + @theme block for custom colors
• Removed tailwind.config.ts (v4 uses CSS-first config; content paths are auto-detected)
• Dropped autoprefixer (handled by @tailwindcss/postcss)
• next build passes cleanly

Test plan

• next build succeeds (verified locally)
• Backend uv sync --group dev installs cleanly
• ruff check . and ruff format --check . pass
• Unit tests pass (pytest tests/ --ignore=tests/test_integration.py)
• CI workflow runs with bumped action versions
• Smoke test multi-agent memory, search, synthesis, and negotiation on oclw3

Closes #37, #38, #39, #40, #41, #42, #43, #44, #45, #46, #47, #48, #49, #50, #51

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