fix(form): keep the Lexicon hint and the "Watching all repos" notice from jumping the layout #95

Follow-up to #87.

Problem #

#87's Expected says "The layout doesn't jump." It still does, in two places under the Lexicon NSID field:

• The hint "Fires when anyone writes a record..." only renders once isValidNsid(lexicon) is true, so typing a valid NSID pushes the rest of the form down, and editing it into an invalid one pulls it back up.
• For an NSID that requires wanted DIDs, the "Watching all repos" notice is removed instead of being reworded, so the form shrinks as soon as the NSID becomes valid.

docs/design.md's "Conditional sections" rule asks for the same thing: content that depends on another field keeps its place and changes its wording instead of appearing and disappearing.

Evidence #

• app/islands/AutomationForm.tsx:3316: {isValidNsid(lexicon) && ( gates the hint.
• app/islands/AutomationForm.tsx:3354-3357: the "Watching all repos" notice, and its branch on isValidNsid(lexicon).
• app/islands/AutomationForm.tsx:1102: nsidRequiresWantedDids decides when the notice is dropped.

Expected #

The text under the Lexicon field takes the same space whatever the NSID is. With no valid NSID yet, it says something generic; with a valid one, it names the NSID; with one that requires wanted DIDs, it says that instead of vanishing.

Acceptance criteria #

• Typing a valid NSID, then making it invalid, doesn't add or remove any block under the field.
• Switching to an NSID that requires wanted DIDs rewords the notice instead of removing it.
• Tests in AutomationForm.test.tsx cover the three cases (blank, valid, requires wanted DIDs).

Out of scope #

Whether the notice should be an alert at all. That's tracked in the "standing Watching all repos alert" issue filed alongside this one.