fix(ux): user-facing error messages (#207, #211, #208)

[dmccoystephenson]

Summary

Replaces developer-facing error copy with plain language:

• Account: login/register failures show developer-speak ("Is the API running?") #207 — account.tsx login/register connection errors no longer say "Is the API running?".
• Leaderboard: API-failure error shows developer-speak ("Is the server running?") #211 — leaderboard.tsx load failure no longer says "Is the server running?".
• Account: registration error always blames "username taken", even when the password is invalid #208 — registration failure no longer speculatively blames the username; it names both the username (availability) and the password requirements.

Test plan

• npm run lint clean, npm run build succeeds.
• Manual trace: each setError(...) path now shows the new copy; technical detail stays in console.error.

Note: the profile/API-key handlers use a plainer "Connection error." — not developer-speak and outside the named issues, so left unchanged.

Closes #207
Closes #211
Closes #208

[dmccoystephenson]

Self-review rubric (CI green):

• Scope: PASS — 2 source files (account, leaderboard) + CHANGELOG; only the named error strings changed.
• Tests-new/fix: N/A — copy changes, no logic; verified by build + tracing each setError path.
• Issue resolution: PASS — all three named strings updated; Account: registration error always blames "username taken", even when the password is invalid #208 no longer mono-blames the username.
• Docs: PASS — CHANGELOG updated.
• CI: PASS. no-any: PASS. mui-only: PASS. env-vars-documented: N/A.
• One judgment call (folded here, no inline): the profile/api-key handlers keep a plainer "Connection error." — not developer-speak and outside Account: login/register failures show developer-speak ("Is the API running?") #207/Leaderboard: API-failure error shows developer-speak ("Is the server running?") #211/Account: registration error always blames "username taken", even when the password is invalid #208's named scope; left to avoid scope creep.