fix: modal scroll, SSH key textarea, nav full-page, remove OS/Provider

- Fix modal scroll: restructure overlay with scroll/centering wrapper
- Add SSH key textarea + file upload to host modal (authType='key')
- Fix textarea reset on new host (clear via DOM)
- SSH username placeholder instead of default 'root'
- Nav uses plain <a href> instead of HTMX for full-page navigation
- /hosts route returns full page; /hosts/list for HTMX partials
- Remove OS and Provider from host modal, grid card, and list card
- Fix key modal x-data scope for Alpine $dispatch
- Update key_grid to display multi-line key content properly
- Simplify SSH key placeholder format

app/backend/views/key_grid.html

@@ -39,7 +39,7 @@
     <div class="flex justify-between items-center py-1 border-b border-outline-variant/10">
       <span class="text-outline font-semibold uppercase tracking-wider text-[10px]">Credential Secrets</span>
       <div class="flex items-center gap-2">
-        <span class="font-mono text-on-surface-variant" x-text="reveal ? '{{.Passphrase}}' : '••••••••••••••••'"></span>
+        <span x-show="!reveal" class="font-mono text-on-surface-variant">••••••••••••••••</span>
         <button @click="reveal = !reveal"
                 class="p-1 hover:bg-surface-container rounded text-outline transition-colors" title="Toggle visibility">
           <svg x-show="reveal" class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13.875 18.825A10.05 10.05 0 0112 19c-4.478 0-8.268-2.943-9.543-7a10.06 10.06 0 011.563-3.029m5.858.908a3 3 0 114.243 4.243M9.878 9.878l4.242 4.242M9.88 9.88L6.59 6.59m7.532 7.532l3.29 3.29M3 3l3.59 3.59m0 0A9.953 9.953 0 0112 5c4.478 0 8.268 2.943 9.543 7a10.025 10.025 0 01-4.132 5.411m0 0L21 21"/></svg>
@@ -51,6 +51,9 @@
         </button>
       </div>
     </div>
+    <div x-show="reveal" x-cloak class="py-1">
+      <code class="text-[10px] text-on-surface-variant font-mono block break-all bg-surface-container-low p-2 rounded border border-outline-variant/20 max-h-32 overflow-y-auto select-all whitespace-pre-wrap">{{.Passphrase}}</code>
+    </div>
 
     {{if .Fingerprint}}
     <div class="py-1">