Encapsulate Legacy Steam Market Behavior and Deprecate JQuery

[GODrums]

Closes #396

Step 1 of #395

Gets rid of JQuery logic in shared functionality in favor of common DOM APIs.
Encapsulates existing Steam Market components to isLegacySteamMarket.

Resolves all errors when using the extension with the Steam Market Beta enabled.

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Medium Risk
Changes shared injection and styling paths used on every market page; incorrect mode detection could skip features on legacy or leave beta issues partially unfixed.

Overview
Adds Steam Market mode detection (legacy vs React beta) and gates legacy-only UI injection and Google Fonts loading so the extension no longer throws on the Market beta.

Injection decorators now accept an optional guard; market utility belt and listing row components only inject when isLegacySteamMarket() is true. Conflict styling in StyleConflictingElement drops jQuery for querySelectorAll and inline setProperty.

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[Step7750]

👍 Lovely edits to the system, thanks!