feat(vim): optional vim-style modal editing in the compose entry

[sypianski]

Closes #535.

What this adds

An optional vim modal editing mode for the message compose entry, enabled by setting vim_mode=1 in ~/.config/nchat/ui.conf. Default is 0 — zero overhead, existing behaviour unchanged.

Commands (normal mode)

Group	Commands
Motions	h l 0 ^ $, w e b W E B, ( ) sentence, { } paragraph, j k, gg G, f F t T
Operators	d c y + any motion; dd cc yy (linewise); D C Y
Text objects	iw aw is as ip ap i" a" i' a' i\ a` i( a( i[ a[ i{ a{ i< a<`
Visual	v → character-wise selection; d c y x; visual text objects (viw, vi", vi( …)
Edit	x X s S o O p P r ~ J
Counts	3w, d3w, 2dd, …
Repeat	. repeats last mutating op (including text-object changes: ciw.)
Clipboard	p/P sync with system clipboard (X11/Wayland/macOS)

Mode is shown as a colored badge in the status bar and the cursor changes shape (bar in insert, block in normal). Badge colors configurable via color.conf (vim_normal_* / vim_insert_* / vim_visual_*).

Other additions in this branch

• Send on double-Enter — send_on_double_enter=1 in ui.conf (default 0): single Enter inserts newline, second consecutive Enter sends

Test plan

• vim_mode=0 (default): no behavioral change, no overhead
• vim_mode=1: ESC enters normal mode, i/a/A/I returns to insert
• Motions move cursor correctly in normal mode
• d/c/y + motion operates on correct range; register available via p
• Text objects: ciw, di", ya( operate correctly
• Visual mode: v starts selection, d/y/c acts on it; viw expands to word
• . repeats last change
• p pastes text copied from outside nchat (clipboard sync)
• Badge color updates on mode change; correct badge in status bar
• send_on_double_enter=1: double Enter sends, single Enter inserts newline

[desgua]

It seems to be working great.
This fix the compilation warnings:

diff --git a/src/uimodel.cpp b/src/uimodel.cpp
index ceb659db..3f6d08f3 100644
--- a/src/uimodel.cpp
+++ b/src/uimodel.cpp
@@ -310,7 +310,6 @@ namespace
   // around=true includes the quote chars (and one trailing or leading space).
   bool VimQuoteObjectRange(const std::wstring& s, int p, wchar_t q, bool around, int& lo, int& hi)
   {
-    int n = (int)s.size();
     int ls = VimLineStart(s, p);
     int le = VimLineEnd(s, p);
     // Find first quote at-or-after line start, count to determine pairing.
@@ -830,7 +829,7 @@ void UiModel::Impl::EntryKeyHandler(wint_t p_Key)
   {
     static const bool sendOnDoubleEnter = (UiConfig::GetNum("send_on_double_enter") == 1);
     wint_t keyLF = 0xA;
-    if (sendOnDoubleEnter && (entryPos > 0) && (entryStr.at(entryPos - 1) == keyLF))
+    if (sendOnDoubleEnter && (entryPos > 0) && (entryStr.at(entryPos - 1) == static_cast<wchar_t>(keyLF)))
     {
       // Second consecutive Enter: drop the just-typed newline and send.
       entryStr.erase(--entryPos, 1);
@@ -1150,6 +1149,8 @@ void UiModel::Impl::VimRepeatLast(int p_Count)
             ok = VimBracketObjectRange(s, pos, L'{', L'}', around, lo, hi); break;
           case L'<': case L'>':
             ok = VimBracketObjectRange(s, pos, L'<', L'>', around, lo, hi); break;
+          default:
+            break;
         }
         if (ok) VimApplyOperator(lc.kind, lo, hi - 1, true, false);
       }
@@ -1169,6 +1170,7 @@ void UiModel::Impl::VimRepeatLast(int p_Count)
         int t = VimComputeMotion(lc.motion, lc.findChar, count, incl, lw, valid);
         if (valid) VimApplyOperator(lc.kind, pos, t, incl, lw);
       }
+    default:
       break;
   }
 }
@@ -1296,7 +1298,7 @@ void UiModel::Impl::VimNormalKey(wint_t p_Key)
         VimApplyOperator(op, lo, hi - 1, true /*inclusive*/, false);
         if ((op == L'd') || (op == L'c') || (op == L'y'))
         {
-          m_VimLast = VimLastChange{op, 1, 0, 0, 0, false, around ? L'a' : L'i', p_Key};
+          m_VimLast = VimLastChange{op, 1, 0, 0, 0, false, static_cast<wint_t>(around ? L'a' : L'i'), p_Key};
         }
       }
     }
@@ -1386,6 +1388,8 @@ void UiModel::Impl::VimNormalKey(wint_t p_Key)
         case L'F': findKey = L'f'; break;
         case L't': findKey = L'T'; break;
         case L'T': findKey = L't'; break;
+        default:
+          break;
       }
     }
     bool incl, lw, valid;