Polish ThreadRelink branding

[ascendho]

Summary

• rebalance the existing ThreadRelink logo so the return arrow is larger and has clear space from the chat bubble
• update the Marketplace/About description to say conversations remain connected to their projects
• document the Azure Free Trial expiry check and subscription-independent Entra fallback
• record the changes under Unreleased without changing the 0.4.1 extension version

Visual changes

The existing palette, gradient, rounded tile, chat bubble, and return-arrow metaphor are preserved. The editable SVG source and packaged PNG were updated together, and the Activity Bar SVG keeps a visible gap between the two symbols at small sizes. README guide screenshots are unchanged.

Publishing impact

This PR does not modify the Marketplace workflows, Azure identity, GitHub environment variables, extension ID, or version. Merging it will not trigger a Marketplace release.

Verification

• pnpm check — lint, typecheck, 27 tests, build
• pnpm package:vscode
• pnpm check:release-version v0.4.1
• verified packaged PNG SHA-256 matches the repository asset
• visually inspected the mark at 1024px, 128px, 32px, and 16px plus the Activity Bar SVG