fix(mobile): show ephemeral (TTL) channels in the channel list

[tlongwell-block]

What

The iOS channel list was unconditionally dropping any channel whose
kind:39000 metadata had a ttl tag, so TTL ("ephemeral") channels were
invisible — even for members, even for the owner.

// mobile/lib/features/channels/channels_provider.dart  (before)
if (!channel.isEphemeral) {
  channels.add(channel);
}

Desktop has no equivalent filter and shows TTL channels with a badge.
Reported when Wes couldn't see #agent-creation-deep-dive on his iOS
app while everyone else saw it fine on desktop.

Why two changes

1) Drop the !isEphemeral filter. channels_page.dart already has
an _EphemeralBadge wired up:

if (channel.isEphemeral) ...[
  const SizedBox(width: Grid.xxs),
  _EphemeralBadge(channel: channel),
],

That branch was dead code because the filter one layer up was
preventing TTL channels from ever reaching it. Someone meant for TTL
channels to show with a badge — the filter was just left in.

2) Parse the archived tag in ChannelData.fromEvent. The relay's
TTL reaper auto-archives expired ephemeral channels and republishes
kind:39000 with ["archived", "true"]. The mobile parser ignored that
tag, leaving Channel.archivedAt permanently null on the Nostr path.
Without this, removing the !isEphemeral guard would let expired TTL
channels stay visible after the reaper hid them. As a bonus, this also
makes user-initiated archive state propagate correctly — the existing
!channel.isArchived filters in channels_page.dart:275 and
channels_provider.dart:207 (_subscribeLive) now work for the Nostr
path the same way they already worked for the JSON path.

The filter has been there since PR #507 introduced the file; no commit
comment explains it. Other channelsProvider consumers (search,
channel management, activity) all handle generic channels — verified
none assume "no ephemeral channels in the list."

Test

Added two regression tests in channels_provider_test.dart:

1. ephemeral (TTL) channels appear in the list — kind:39000 with
   ["ttl", "86400"] → channel surfaces with isEphemeral=true.
2. archived kind:39000 metadata sets Channel.isArchived (covers TTL auto-archive) — kind:39000 with both ttl and ["archived","true"]
   → channel surfaces with isArchived=true and isEphemeral=true, so
   the existing _SliverChannelsList filter hides it. Sibling active
   channel stays unarchived.

flutter test test/features/channels/channels_provider_test.dart
00:00 +5: All tests passed!
flutter test            # full mobile suite
00:06 +332 ~1: All tests passed!
flutter analyze
No issues found!

Risk

Tiny. Two narrow changes to mobile parsing, no relay-side behavior
changed, regression tests pin both the active-TTL and archived-TTL
cases. Existing test for "subscribes only to joined, non-archived"
still passes — proves _subscribeLive's archive filter continues to
work.

Credit

The archived-tag oversight was caught by a codex review pass on
the first revision of this PR — without it, this would have shipped a
visible regression once the first TTL channel expired.