QoL: rebuild the teleport tables if they came up empty at login

[dfrisone]

Fix from Svart. His GitHub account is currently suspended, so this is being submitted on his behalf.

What does this PR do?

Fixes dungeon teleport buttons in the /ekeys popup being unclickable/hidden.

MAP_TELEPORT_SPELLS and TELEPORT_BY_NAME were built exactly once at file load from C_ChallengeMode/GetLFGDungeonInfo data. If that data was not cached yet at that moment on login, both tables stayed permanently empty for the whole session, and every teleport button stayed hidden with no way to recover short of a reload.

The build is extracted into RebuildTeleportTables() and re-run from ShowKeystonePopup() whenever the tables are still empty, so a late-populated cache self-heals the next time /ekeys is opened.

How was it tested?

Author-tested by Svart; not independently retested in-game by me before opening this on his behalf.

Checklist

• New settings default OFF (no behavior change without opt-in) -- N/A, no new settings
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built -- the self-heal check is two table reads inside a function that already runs when the popup opens; the rebuild itself only runs when the tables are genuinely empty
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations) -- N/A, no new events
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames -- N/A, no frame code touched
• Tested in-game on live; no version gates or pre-Midnight APIs added -- see note above