refactor(extension): remove dead MCP toggle plumbing

[FlyM1ss]

What

MCP is now the default for every agent, so the client-side "MCP Mode" toggle is obsolete. Its UI was already disabled (commented out) in an earlier cleanup, which meant useMCP was hardwired to false at every call site. This PR removes the resulting dead plumbing.

Why now

The toggle predates MCP being always-on. Because document.getElementById('mcpModeSwitch') returns null (the control is commented out), useMCP could never be true. The leftover code was actively misleading:

• an unreachable if (useMCP) branch in getChatResponseStream that routed to the non-streaming path — i.e. it would have disabled streaming if the toggle ever came back;
• a "MCP mode doesn't support streaming yet" comment describing code that can never run;
• a !useMCP term in the streaming gate and a 'MCP mode' status label that can never show.

Changes

• api.js — drop the useMCP param from getChatResponse / getChatResponseStream; delete the unreachable if (useMCP) non-streaming branch; correct the endpoint comment.
• handlers.js — drop the mcpModeSwitch read, the !useMCP term in canStream, the 'MCP mode' detail label, and the useMCP argument at all call sites (including createActionButtons).
• settings_window.js — delete the commented-out MCP toggle tombstone.

Scope / safety

• No behavior change. useMCP was provably always false, so no reachable path is altered.
• RAG left untouched — it's a separately parked feature (also currently commented out).
• Base: stacked on security/extension-post-sse (feat(extension): migrate chat endpoints to POST fetch-streaming (v0.16.0) #335) because it edits the same post-migration api.js; GitHub will retarget this to main once feat(extension): migrate chat endpoints to POST fetch-streaming (v0.16.0) #335 merges.

Verification

• bun test — 55 pass, no new failures.
• bun run build:full — webpack compiles clean; dist/main.js contains zero useMCP / mcpModeSwitch references.

⚠️ Note (pre-existing, out of scope): 2 markdownRenderer DOMPurify XSS-sanitizer tests fail identically on the base branch security/extension-post-sse with this diff stashed. They live in code this PR never touches. Flagging so it isn't mistaken for a regression here.

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