fix(profile): the handle's hint says it is private until the profile is public (ADR 0020, D5)

A handle alone is private: its page and `profiles.byHandle` answer as
for a handle nobody has until its person fills in their profile or
publishes something. The hint under it said "Others find you as @…"
all the same, also after removing a profile (D27). It now says it is
private until then, by the rule the instance's `isPublic` applies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -550,6 +550,8 @@
   "publicProfileHandleHint": "Letters, digits and underscores, up to 30",
   "publicProfileAddress": "Others find you as @{address}",
   "@publicProfileAddress": {"placeholders": {"address": {"type": "String"}}},
+  "publicProfileAddressPrivate": "Private until you fill in your profile or publish something; then others find you as @{address}.",
+  "@publicProfileAddressPrivate": {"placeholders": {"address": {"type": "String"}}},
   "publicProfileHandleFixed": "Fixed since you published: others refer to it.",
   "publicProfileBio": "Bio",
   "publicProfileLinks": "Links",