fix(core): validate fxserver command input and simplify dispatch

[mattibat]

Problem

• action/parameter were only checked for "defined", not type
• Command dispatch was a long if/else chain with permission check duplicated per branch, easy to forget on a new action

Fix

• action/parameter now require typeof === 'string'
• Dispatch replaced with a lookup table: action -> { permission, handler }
• Permission check centralized in one place
• No behavior change beyond the stricter input validation

[tabarra]

Please don't spam AI-generated PRs, this one is also just noise that shuffles code around for little to no gain.
This is one of the oldest files in txAdmin, sure it can be better, but:

1. Any change brings new risks
2. It makes harder to do a git blame drill down
3. Makes this file different than its sibling controls.ts

The "problem" you described has literally zero impact, an intentionally crafted invalid authenticated request would cause a zero-impact reflected 500 error if the params exist but not string, and your code solves this bug, but it also introduces the exact same bug again for strings like toString, and constructor... so what was gained here?
What could be a two-lined patch became a full file rewrite.

And again, I do recognize that this file could be done better, but so can basically every file in this codebase.
I could also ask Codex or Claude to go around the code doing drive-by refactors... but I just don't want to.

[mattibat]

fair point. agreed the refactor wasn't worth the diff for what it fixed. Happy to send just the 2 line validation fix on its own if thats useful, otherwise no worries either way.