Add form validation feedback in image URL builder

[bonheur15]

Problem

The image URL builder form on the landing page performs no input validation beyond checking for an empty source URL. Invalid inputs produce no visual error message.

Location: internal/web/views/landing.templ:90-143

Missing Validation

Input	HTML Attribute	Issue
<input type="text" id="b-src">	No required, no pattern	Empty/invalid URLs silently show placeholder text
<input type="number" id="b-w" min="1" max="4096">	min/max set but no custom error	Values outside range show browser default tooltip
<input type="number" id="b-q" min="1" max="100">	min/max set but no custom error	Same as above

The JavaScript build() function at lines 115-143 performs no validation beyond if (!u) for empty source.

Proposed Fix

1. Add visual error styling (red border) for invalid inputs
2. Show inline error messages below or beside invalid fields
3. Validate URL format before attempting to load the preview
4. Add required attributes to mandatory fields
5. Consider using the Constraint Validation API for custom messages

Example

function validate() {
    let valid = true;
    const src = document.getElementById("b-src");
    const w = document.getElementById("b-w");
    const q = document.getElementById("b-q");
    
    if (!src.value) {
        src.classList.add("error");
        valid = false;
    } else {
        src.classList.remove("error");
    }
    
    if (w.value && (w.value < 1 || w.value > 4096)) {
        w.classList.add("error");
        valid = false;
    } else {
        w.classList.remove("error");
    }
    
    // ... similar for q
    return valid;
}

.builder input.error {
    border-color: #d73a49;
    background: #ffeef0;
}