Add Organizations' User Invitation UI

[yasinelmi]

Summary

Implements the frontend for #6088: a My Organizations landing page, an organization
edit page with Details and Sharing tabs (invite/manage members), and organization
creation — following the existing channel edit/details page pattern.

Closes #6088

New pages and features

Screenshots below are placeholders — see "How to reproduce these screenshots" underneath
for exactly what to capture. Log in as a@a.com / a on a local pnpm devsetup instance
(seeded via python manage.py setup, which now also creates two sample organizations and
a pending invitation).

Description	Screenshot
My Organizations landing page — reached via the new "My organizations" nav tab. Shows a banner of pending organization invitations (accept/decline) above a card grid of the organizations you belong to, each showing your role in it.	(screenshot: /#/my-organizations)
Organization card options menu — the "⋮" button on each card opens a dropdown with "Edit organization," which goes to the same edit page as clicking the card body.	(screenshot: click the ⋮ on any org card)
New organization form — reached via "New organization" on the landing page. Same page/component as editing an existing org, just blank, with no tabs (nothing to share yet) and a "Create organization" button instead of "Save changes." The creator automatically becomes the org's admin.	(screenshot: click "New organization")
Organization edit page — Details tab (as admin) — name/description/public checkbox, editable. Header uses the same dark "immersive modal" banner as the channel details page, including its focus trap and Escape-to-close behavior.	(screenshot: open an org you're admin of → Details tab)
Organization edit page — Details tab (as a non-admin member) — same page, but fields are disabled with an explanatory notice ("Only organization admins can edit these details") instead of silently failing on save.	(screenshot: open an org where your role is Editor/Viewer)
Sharing tab — invite + manage members — only visible to org admins. An invite-by-email form with a role selector (Viewer/Editor/Admin), plus a single table unifying active members and pending invitations, each with a "⋮" menu (role change / remove for members; resend / revoke for pending invites).	(screenshot: Sharing tab, as admin)
Sharing tab — as a non-admin — replaced entirely with a "Only organization admins can manage sharing settings" message; no invite form or member table rendered at all.	(screenshot: Sharing tab, as Editor/Viewer)
Remove-member / revoke-invitation confirmation — both are destructive actions gated behind a confirmation modal naming the affected email.	(screenshot: ⋮ → "Remove from organization" or "Revoke invitation" on a row)
Last-active-admin protection — the API refuses to demote, deactivate, or remove an organization's only active admin; the UI surfaces the exact server error as a snackbar instead of failing silently.	(screenshot: as the sole admin, try "Make viewer" or "Remove from organization" on yourself)

Framework compliance (per the issue's Frameworks checklist)

• No Vuetify anywhere in the new code — every component uses Kolibri Design System
  (KCard, KButton, KTextbox, KCheckbox, KSelect, KTable, KTabsList,
  KModal, KDropdownMenu, StudioImmersiveModal).
• No Vuex — all new state is Composition API composables
  (useOrganization, useOrganizationList, useOrganizationMembers,
  useOrganizationInvitations).

Accessibility

• Every icon-only button has an ariaLabel/tooltip; the options button on each
  organization card carries a per-card distinguishing label ("More options for
  {org name}") rather than a generic repeated one.
• Keyboard focus is trapped inside the edit page while open (via StudioImmersiveModal's
  KFocusTrap), with Escape closing it — matching the existing channel details page.
• The document/tab title updates on every route and tab change, so screen readers get a
  cue that the SPA navigated even though there's no real page reload.
• Disabled form fields for non-admins are paired with visible explanatory text, not just
  a visual dimming.

Test plan

• pytest — organization, organization-role-annotation, invitation, and channel
  viewset tests pass (219 backend tests)
• pnpm test — full channelList frontend suite passes (226 tests)
• pre-commit run --all-files on all touched files
• Manually verified in the browser: invitation accept/decline, invite-by-email, role
  changes, resend/revoke invites, member removal (including the last-active-admin
  guard), organization creation, and the non-admin read-only states

Notes for reviewers

• No Vuex and no Vuetify anywhere in the new code (all new state is Composition API
  composables) per the issue's Frameworks requirements.
• Draft PR — opening for visibility into the current state of the changes while this is
  still being reviewed/iterated on. Screenshots to be added shortly.

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