feat: per-plugin & per-skill cost attribution

[vivekchand]

Closes #201

What

Adds a /api/cost-attribution endpoint and a visual cost attribution panel in the Tokens tab that breaks down token usage and cost by tool category (Reasoning, File & Shell, Web Research, Browser Automation, Communication, Sub-Agents, Media & Analysis, System).

How

• Backend: New @bp_usage.route("/api/cost-attribution") parses session JSONL files and attributes each assistant turn's cost to the tools used in that turn (equal split when multiple tools). Turns with no tool calls are categorized as "Reasoning". Results are cached for 30s.
• Frontend: SVG donut chart showing cost distribution by category with a legend, expandable per-tool breakdown table. Added to both dark and light theme HTML sections.
• Tests: 5 new test cases in tests/test_api.py covering endpoint accessibility, response structure, category/tool field validation, and daily breakdown presence.

Categories

Category	Tools
Reasoning	Text-only turns (no tool calls)
File & Shell	read, write, edit, exec
Web Research	web_search, web_fetch
Browser Automation	browser
Communication	message, tts
Sub-Agents	sessions_spawn, sessions_send, subagents, etc.
Media & Analysis	image, pdf, canvas
System	session_status, agents_list, nodes, process