Nicknames: source-picker favourites + B.net cross-session fix

[Maelareth]

Summary

Two things, both in the Nicknames feature:

1. Source-picker favourites — a quality-of-life upgrade to the "Add from" pickers.
2. A B.net rules bug fix — Battle.net nickname rules were silently breaking across sessions.

Favourites (feature)

• Star any group / guild / friend / Battle.net member in the "Add from" picker to pin them to the top.
• Each list sorts into tiers — favourites → online → offline (A–Z within each) — with a divider between the favourites and the rest.
• Stored account-wide. Characters share one favourites list across the group/guild/friends pickers; Battle.net is keyed by BattleTag.

B.net cross-session fix (bug)

• B.net rules previously stored the session-only bnetAccountID, so after a full relog the id changed and the rule silently stopped applying — the rule stayed in the list but the nickname vanished. Confirmed against the API docs (bnetAccountID is documented as a temporary, session-scoped id) and a code review.
• Rules now key off the stable BattleTag. Existing rules are migrated where possible (via the still-valid session id, or by matching the stored label to an account name / BattleTag); any that can't be resolved are flagged in the list ("needs re-link") so they can be re-added rather than silently lost.

Also included

• Perf: a per-GUID resolve cache (wiped in RefreshAllFrames) removes repeated string work on the name path.
• Hardening: the self-broadcast name is built with GetNormalizedRealmName() for realm-spelling parity with the addon-message sender string; the received-nickname short-name key no longer collides across realms.

Testing

• Favourites: starring/un-starring, the 3-tier sort, the divider, and the shared list across friends/guild verified in-game across all four pickers.
• B.net: adding friends (now stores BattleTag) and the migration path verified; no Lua errors.
• Existing nickname behaviour regression-checked.

[Maelareth]

Pushed one more commit (b1a36b2) after testing the B.net path across a full relog.

What it fixes: a B.net rule's display label was being saved from C_BattleNet.GetFriendAccountInfo().accountName, which is a session-scoped |K…|k Battle.net name token rather than plain text. It renders to the friend's current name client-side, and the index inside it is reassigned each login — so after a relog the Saved-nicknames list showed a different friend's name for the rule (e.g. Pinkus → RakbladeT), even though the rule still matched the correct account.

The BattleTag-based matching was already correct (nicknames applied to the right people); only the persisted label drifted. The commit:

• derives the rule's display label from the stable BattleTag (name part), never from accountName;
• adds a one-time repair that rewrites any label already saved as a |K…|k token on load;
• stops the legacy migration from matching a token pattern (which could otherwise mis-assign a different friend).

Worth noting both this and the original cross-session matching bug exist in the current alpha, so this PR fixes two real B.net issues alongside the favourites feature.