Persist editor state in localStorage across reloads

[MrBeldum]

Goal

Remember the last username, message, color, and selected built-in badges when the page reloads, so a refresh does not wipe in-progress work.

Scope

• On meaningful state changes, save a small JSON blob to localStorage (version the key, e.g. tmc-state-v1).
• On load, restore username, message, color (via parseHexColor), and selected built-in badge IDs that still exist in BADGES.
• Reset should clear persisted state as well as the UI.
• Fail soft: corrupt/missing storage must fall back to current defaults without breaking the page.

Out of scope

• Persisting custom badge image data URLs (can be large; skip or document as follow-up).
• Syncing across tabs beyond a simple reload restore.
• New dependencies or frameworks.

Acceptance

• Reload restores the last valid username/message/color/built-in badges.
• Reset clears storage and returns to defaults.
• Invalid stored color is ignored (defaults apply).
• node --test still passes; manual check via ./start-local.sh.

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