feat(settings): /settings/workspace — edit company info + invoice defaults

Owner/admin edit, member read-only.

Schema/validation:
- lib/validation/workspace.ts (workspaceSettingsSchema)
- lib/appwrite/workspace-actions.ts:
  * updateWorkspaceSettingsAction — requireRole owner|admin, upserts the
    tenant_settings row (creates one with team-scoped perms if absent,
    e.g. for tenants created before tenant_settings was a table; just
    defense). Audit-logged.
  * Forces invoicePrefix to uppercase. defaultVatRate clamped to [0, 100].
  * revalidatePath('/', 'layout') so the new company name updates in
    sidebar header and dashboard greeting on next render.

UI:
- /settings/workspace page (server) — pulls active tenant settings
  via requireTenant, shows form pre-filled.
- WorkspaceSettingsForm: 2 cards
  * Şirket — name (required), tax id, phone, email, address
  * Faturalama — invoicePrefix, defaultVatRate, read-only invoiceCounter
- All inputs disabled if user is a member (canEdit=false). Submit button
  hidden in that case. Description on the page changes accordingly.
- Toast feedback for success/error.

Skipped: logo upload (storage bucket pending). Will revisit.

src/lib/validation/workspace.ts

@@ -0,0 +1,44 @@
+import { z } from "zod";
+
+export const workspaceSettingsSchema = z.object({
+  companyName: z.string().trim().min(1, "Şirket adı zorunlu.").max(255),
+  companyTaxId: z
+    .string()
+    .trim()
+    .max(50)
+    .optional()
+    .transform((v) => (v ? v : undefined)),
+  companyAddress: z
+    .string()
+    .trim()
+    .max(500)
+    .optional()
+    .transform((v) => (v ? v : undefined)),
+  companyEmail: z
+    .union([z.string().email("Geçerli bir email girin."), z.literal("")])
+    .optional()
+    .transform((v) => (v ? v : undefined)),
+  companyPhone: z
+    .string()
+    .trim()
+    .max(30)
+    .optional()
+    .transform((v) => (v ? v : undefined)),
+  defaultVatRate: z
+    .union([z.number(), z.string()])
+    .optional()
+    .transform((v) => {
+      if (v === undefined || v === "") return 20;
+      const n = typeof v === "string" ? Number(v.replace(",", ".")) : v;
+      return Number.isFinite(n) ? n : 20;
+    })
+    .pipe(z.number().min(0, "Negatif olamaz.").max(100, "100'den büyük olamaz.")),
+  invoicePrefix: z
+    .string()
+    .trim()
+    .max(10)
+    .optional()
+    .transform((v) => (v ? v.toUpperCase() : "INV")),
+});
+
+export type WorkspaceSettingsInput = z.infer<typeof workspaceSettingsSchema>;