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/nav.html

@@ -1,6 +1,6 @@
 {{define "nav"}}
 <nav class="w-[260px] min-h-screen bg-white/70 backdrop-blur-md border-r border-outline-variant/30 flex flex-col shrink-0"
-     x-data="{ page: '{{.View}}' }" @htmx:after-settle.window="page = location.pathname.slice(1) || 'hosts'">
+     x-data="{ page: '{{.View}}' }">
   <div class="p-3 border-b border-outline-variant/20">
     <div class="flex items-center gap-2">
       <div class="w-8 h-8 rounded-lg bg-primary flex items-center justify-center text-white text-sm font-bold">K</div>
@@ -10,7 +10,6 @@
   <div class="flex-1 px-3 py-4 space-y-0.5">
     {{range .NavItems}}
     <a href="/{{.ID}}"
-       hx-get="/{{.ID}}" hx-target="#main-content" hx-push-url="true"
        :class="page === '{{.ID}}' ? 'bg-primary/10 text-primary font-medium' : 'text-on-surface-variant hover:bg-surface-container-low hover:text-on-surface'"
        class="flex items-center gap-3 px-3 py-2 rounded-lg transition-colors text-sm">
       <span class="w-5 h-5 flex items-center justify-center">{{svg .Icon}}</span>