client create: auto-generate the client name (<firstname>-NN) and make --location optional

[LukasWodka]

Part of the installer first-run UX v2 spec (umbrella in the backend tracker; interim copy fixes in tracebloc/client#307). Goal: client create needs zero flags and zero prompts on the installer path — name auto-generated, location omitted.

Auto-generated client name — <firstname>-NN

• Parse first_name from GET /userinfo/ into api.Identity and persist it in the profile at login (the backend already returns it for account users; fallback when empty: email local-part).
• When --name is absent, auto-name the client <slug(first_name)>-NN — lukas-01, lukas-02, … NN = next free number scanned from the account's existing client names (ListClients already fetches the full paginated list in runClientCreate). Zero-padded 2 digits.
• No name prompt, ever. --name / TRACEBLOC_CLIENT_NAME still override.
• The auto-name passes internal/slug untouched (lukas-01 → lukas-01), so display name = namespace = handle by construction, and the -2 slug collision suffix never fires for auto-names.
• Note: this is a deliberate product deviation from RFC-0001 §6.6 (which derives from the hostname) — decided by Lukas 2026-07-06. Update the RFC §6.6/§7.7 wording in a companion docs commit.

--location optional

• Remove the location requirement: no errMissingFlag("--location"), no prompt (delete the validateNonEmpty prompt block).
• CreateClientRequest.Location → json:"location,omitempty" (the backend accepts absent/blank today — EdgeDevice.location is blank=True, serializer auto-builds required=False).
• Guard the review render (renderClientReview) for empty location; resumeCommand is already conditional.
• The internal/geo auto-detect (cli#93) stays available for --location-less enrichment later but must not block or prompt.

Noun pass (naming lock)

• Adopt message: "This cluster is already registered as client …" → machine/client nouns per the naming rule (machine = hardware, client = tracebloc entity).
• Sweep remaining user-facing strings for vendor → other collaborators (installer strings already swept in client#307).

Acceptance

• tracebloc client create --yes --credential-file f with no other flags, non-interactive: succeeds — auto-name minted, no location sent.
• Two machines, same account, same first name → lukas-01, lukas-02 (numbering, not slug -2 stacking).
• Re-run on the same cluster still adopts (idempotency unchanged).
• --name/--location flags still honored verbatim.

Refs: RFC-0001 (cli#55) §6.6/§6.7/§7.7 · connect/install flow family backend#877.

🤖 Generated with Claude Code

[LukasWodka]

Umbrella spec: tracebloc/backend#992 (installer first-run UX v2). Sibling: #138. Interim installer copy: tracebloc/client#307.