Fix Discourse deprecation warnings (hbs-extension, resolver-resolutions, static-viewport)

[mavi-sqr]

Why

Admins on community.seqera.io (Discourse 2026.9.0-latest) see three recurring
"Theme 'Clean' contains code which needs updating" warnings:

• discourse.hbs-extension
• discourse.resolver-resolutions
• discourse.static-viewport-initialization

These are Discourse deprecations (not the Dependabot/npm audit findings — those
are dev-only lint tooling that never ships to the forum). .hbs support is being
removed in the 2026.8+ line, so this is time-sensitive.

What

• Convert clean-user-info.hbs → .gjs with the correct core imports for 2026.9
  (discourse/ui-kit/helpers/*, discourse-i18n). Dates use dFormatDate, whose
  medium/title defaults replicate the old {{bound-date}} output exactly — and
  bound-date is itself now deprecated, so this avoids trading one warning for another.
• Delete the dead navigation-bar.hbs template override. Template overrides were
  removed from core in June 2025; the file only emitted warnings (its content was the
  default nav items plus the already-existing extra-nav-item outlet). Clears both
  hbs-extension and resolver-resolutions for that file.
• Drop the deprecated site.mobileView lookup in init-nav-controls.js. The
  mobile-only hide-on-scroll effect stays correctly scoped via the existing
  @media (max-width: 780px) block in common.scss (verified — desktop is unaffected).

Also cleaned up a mixed package-manager state (stray package-lock.json removed;
yarn.lock restored).

Note on why not merge upstream Lhcfl

Merging upstream would fix only 2 of 3 warnings, and destructively: upstream deleted
the custom user-info panel (moved to a separate theme-component) and never fixed the
static-viewport call. Histories have also diverged (15 ahead / 99 behind). Surgical
fixes here clear all three while keeping the Seqera branding.

Verification

• ember-template-lint and prettier pass clean on the changed files.
• Before merge, test on a Discourse dev instance: import the theme, confirm
  Admin → Customize → Themes → Clean shows no deprecation banners for the three IDs,
  and check the user-profile panel + navigation render identically (mobile: nav hides on
  scroll; desktop: unaffected).

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